diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index dd2480a73d..480cc0bd8c 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -629,14 +629,21 @@ static off_t write_reuse_object(struct hashfile *f, struct object_entry *entry,
 	struct packed_git *p = IN_PACK(entry);
 	struct pack_window *w_curs = NULL;
 	uint32_t pos;
-	off_t offset;
+	off_t offset, cur;
 	enum object_type type = oe_type(entry);
+	enum object_type in_pack_type;
 	off_t datalen;
 	unsigned char header[MAX_PACK_OBJECT_HEADER],
 		      dheader[MAX_PACK_OBJECT_HEADER];
 	unsigned hdrlen;
 	const unsigned hashsz = the_hash_algo->rawsz;
-	unsigned long entry_size = SIZE(entry);
+	size_t entry_size;
+
+	cur = entry->in_pack_offset;
+	in_pack_type = unpack_object_header(p, &w_curs, &cur, &entry_size);
+	if (in_pack_type < 0)
+		die(_("write_reuse_object: unable to parse object header of %s"),
+		    oid_to_hex(&entry->idx.oid));
 
 	if (DELTA(entry))
 		type = (allow_ofs_delta && DELTA(entry)->idx.offset) ?
@@ -664,7 +671,8 @@ static off_t write_reuse_object(struct hashfile *f, struct object_entry *entry,
 	datalen -= entry->in_pack_header_size;
 
 	if (!pack_to_stdout && p->index_version == 1 &&
-	    check_pack_inflate(p, &w_curs, offset, datalen, entry_size)) {
+	    check_pack_inflate(p, &w_curs, offset, datalen,
+			       cast_size_t_to_ulong(entry_size))) {
 		error(_("corrupt packed object for %s"),
 		      oid_to_hex(&entry->idx.oid));
 		unuse_pack(&w_curs);
@@ -1087,7 +1095,7 @@ static void write_reused_pack_one(struct packed_git *reuse_packfile,
 {
 	off_t offset, next, cur;
 	enum object_type type;
-	unsigned long size;
+	size_t size;
 
 	offset = pack_pos_to_offset(reuse_packfile, pos);
 	next = pack_pos_to_offset(reuse_packfile, pos + 1);
@@ -2243,7 +2251,7 @@ static void check_object(struct object_entry *entry, uint32_t object_index)
 		off_t ofs;
 		unsigned char *buf, c;
 		enum object_type type;
-		unsigned long in_pack_size;
+		size_t in_pack_size;
 
 		buf = use_pack(p, &w_curs, entry->in_pack_offset, &avail);
 
@@ -2270,7 +2278,7 @@ static void check_object(struct object_entry *entry, uint32_t object_index)
 		default:
 			/* Not a delta hence we've already got all we need. */
 			oe_set_type(entry, entry->in_pack_type);
-			SET_SIZE(entry, in_pack_size);
+			SET_SIZE(entry, cast_size_t_to_ulong(in_pack_size));
 			entry->in_pack_header_size = used;
 			if (oe_type(entry) < OBJ_COMMIT || oe_type(entry) > OBJ_BLOB)
 				goto give_up;
@@ -2324,8 +2332,8 @@ static void check_object(struct object_entry *entry, uint32_t object_index)
 		if (have_base &&
 		    can_reuse_delta(&base_ref, entry, &base_entry)) {
 			oe_set_type(entry, entry->in_pack_type);
-			SET_SIZE(entry, in_pack_size); /* delta size */
-			SET_DELTA_SIZE(entry, in_pack_size);
+			SET_SIZE(entry, cast_size_t_to_ulong(in_pack_size)); /* delta size */
+			SET_DELTA_SIZE(entry, cast_size_t_to_ulong(in_pack_size));
 
 			if (base_entry) {
 				SET_DELTA(entry, base_entry);
@@ -2734,16 +2742,18 @@ unsigned long oe_get_size_slow(struct packing_data *pack,
 	struct pack_window *w_curs;
 	unsigned char *buf;
 	enum object_type type;
-	unsigned long used, avail, size;
+	unsigned long used, avail;
+	size_t size;
 
 	if (e->type_ != OBJ_OFS_DELTA && e->type_ != OBJ_REF_DELTA) {
+		unsigned long sz;
 		packing_data_lock(&to_pack);
 		if (odb_read_object_info(the_repository->objects,
-					 &e->idx.oid, &size) < 0)
+					 &e->idx.oid, &sz) < 0)
 			die(_("unable to get size of %s"),
 			    oid_to_hex(&e->idx.oid));
 		packing_data_unlock(&to_pack);
-		return size;
+		return sz;
 	}
 
 	p = oe_in_pack(pack, e);
@@ -2760,7 +2770,7 @@ unsigned long oe_get_size_slow(struct packing_data *pack,
 
 	unuse_pack(&w_curs);
 	packing_data_unlock(&to_pack);
-	return size;
+	return cast_size_t_to_ulong(size);
 }
 
 static int try_delta(struct unpacked *trg, struct unpacked *src,
diff --git a/object-file.c b/object-file.c
index 086b2b65ff..0be2981c7a 100644
--- a/object-file.c
+++ b/object-file.c
@@ -2326,6 +2326,7 @@ int odb_source_loose_read_object_stream(struct odb_read_stream **out,
 	struct object_info oi = OBJECT_INFO_INIT;
 	struct odb_loose_read_stream *st;
 	unsigned long mapsize;
+	unsigned long size_ul;
 	void *mapped;
 
 	mapped = odb_source_loose_map_object(source, oid, &mapsize);
@@ -2349,11 +2350,18 @@ int odb_source_loose_read_object_stream(struct odb_read_stream **out,
 		goto error;
 	}
 
-	oi.sizep = &st->base.size;
+	/*
+	 * object_info.sizep is unsigned long* (32-bit on Windows), but
+	 * st->base.size is size_t (64-bit). Use temporary variable.
+	 * Note: loose objects >4GB would still truncate here, but such
+	 * large loose objects are uncommon (they'd normally be packed).
+	 */
+	oi.sizep = &size_ul;
 	oi.typep = &st->base.type;
 
 	if (parse_loose_header(st->hdr, &oi) < 0 || st->base.type < 0)
 		goto error;
+	st->base.size = size_ul;
 
 	st->mapped = mapped;
 	st->mapsize = mapsize;
diff --git a/odb/streaming.c b/odb/streaming.c
index 5927a12954..af2adf5ce7 100644
--- a/odb/streaming.c
+++ b/odb/streaming.c
@@ -157,15 +157,26 @@ static int open_istream_incore(struct odb_read_stream **out,
 		.base.read = read_istream_incore,
 	};
 	struct odb_incore_read_stream *st;
+	unsigned long size_ul;
 	int ret;
 
 	oi.typep = &stream.base.type;
-	oi.sizep = &stream.base.size;
+	/*
+	 * object_info.sizep is unsigned long* (32-bit on Windows), but
+	 * stream.base.size is size_t (64-bit). We use a temporary variable
+	 * because the types are incompatible. Note: this path still truncates
+	 * for >4GB objects, but large objects should use pack streaming
+	 * (packfile_store_read_object_stream) which handles size_t properly.
+	 * This incore fallback is only used for small objects or when pack
+	 * streaming is unavailable.
+	 */
+	oi.sizep = &size_ul;
 	oi.contentp = (void **)&stream.buf;
 	ret = odb_read_object_info_extended(odb, oid, &oi,
 					    OBJECT_INFO_DIE_IF_CORRUPT);
 	if (ret)
 		return ret;
+	stream.base.size = size_ul;
 
 	CALLOC_ARRAY(st, 1);
 	*st = stream;
diff --git a/odb/streaming.h b/odb/streaming.h
index c7861f7e13..517e2ea2d3 100644
--- a/odb/streaming.h
+++ b/odb/streaming.h
@@ -21,7 +21,7 @@ struct odb_read_stream {
 	odb_read_stream_close_fn close;
 	odb_read_stream_read_fn read;
 	enum object_type type;
-	unsigned long size; /* inflated size of full object */
+	size_t size; /* inflated size of full object */
 };
 
 /*
diff --git a/oss-fuzz/fuzz-pack-headers.c b/oss-fuzz/fuzz-pack-headers.c
index 150c0f5fa2..ef61ab577c 100644
--- a/oss-fuzz/fuzz-pack-headers.c
+++ b/oss-fuzz/fuzz-pack-headers.c
@@ -6,7 +6,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
 	enum object_type type;
-	unsigned long len;
+	size_t len;
 
 	unpack_object_header_buffer((const unsigned char *)data,
 				    (unsigned long)size, &type, &len);
diff --git a/pack-bitmap.c b/pack-bitmap.c
index f6ec18d83a..f9af8a96bd 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -2270,7 +2270,7 @@ static int try_partial_reuse(struct bitmap_index *bitmap_git,
 {
 	off_t delta_obj_offset;
 	enum object_type type;
-	unsigned long size;
+	size_t size;
 
 	if (pack_pos >= pack->p->num_objects)
 		return -1; /* not actually in the pack */
diff --git a/pack-check.c b/pack-check.c
index 79992bb509..2792f34d25 100644
--- a/pack-check.c
+++ b/pack-check.c
@@ -110,7 +110,7 @@ static int verify_packfile(struct repository *r,
 		void *data;
 		struct object_id oid;
 		enum object_type type;
-		unsigned long size;
+		size_t size;
 		off_t curpos;
 		int data_valid;
 
@@ -143,7 +143,9 @@ static int verify_packfile(struct repository *r,
 			data = NULL;
 			data_valid = 0;
 		} else {
-			data = unpack_entry(r, p, entries[i].offset, &type, &size);
+			unsigned long sz;
+			data = unpack_entry(r, p, entries[i].offset, &type, &sz);
+			size = sz;
 			data_valid = 1;
 		}
 
diff --git a/packfile.c b/packfile.c
index b012d648ad..fdae91dd11 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1133,7 +1133,7 @@ out:
 }
 
 unsigned long unpack_object_header_buffer(const unsigned char *buf,
-		unsigned long len, enum object_type *type, unsigned long *sizep)
+		unsigned long len, enum object_type *type, size_t *sizep)
 {
 	unsigned shift;
 	size_t size, c;
@@ -1144,7 +1144,11 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 	size = c & 15;
 	shift = 4;
 	while (c & 0x80) {
-		if (len <= used || (bitsizeof(long) - 7) < shift) {
+		/*
+		 * Each continuation byte adds 7 bits. Ensure shift won't
+		 * overflow size_t (use size_t not long for 64-bit on Windows).
+		 */
+		if (len <= used || (bitsizeof(size_t) - 7) < shift) {
 			error("bad object header");
 			size = used = 0;
 			break;
@@ -1153,7 +1157,7 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 		size = st_add(size, st_left_shift(c & 0x7f, shift));
 		shift += 7;
 	}
-	*sizep = cast_size_t_to_ulong(size);
+	*sizep = size;
 	return used;
 }
 
@@ -1215,7 +1219,7 @@ unsigned long get_size_from_delta(struct packed_git *p,
 int unpack_object_header(struct packed_git *p,
 			 struct pack_window **w_curs,
 			 off_t *curpos,
-			 unsigned long *sizep)
+			 size_t *sizep)
 {
 	unsigned char *base;
 	unsigned long left;
@@ -1367,7 +1371,7 @@ static enum object_type packed_to_object_type(struct repository *r,
 
 	while (type == OBJ_OFS_DELTA || type == OBJ_REF_DELTA) {
 		off_t base_offset;
-		unsigned long size;
+		size_t size;
 		/* Push the object we're going to leave behind */
 		if (poi_stack_nr >= poi_stack_alloc && poi_stack == small_poi_stack) {
 			poi_stack_alloc = alloc_nr(poi_stack_nr);
@@ -1586,7 +1590,7 @@ static int packed_object_info_with_index_pos(struct packed_git *p, off_t obj_off
 					     uint32_t *maybe_index_pos, struct object_info *oi)
 {
 	struct pack_window *w_curs = NULL;
-	unsigned long size;
+	size_t size;
 	off_t curpos = obj_offset;
 	enum object_type type = OBJ_NONE;
 	uint32_t pack_pos;
@@ -1778,7 +1782,7 @@ void *unpack_entry(struct repository *r, struct packed_git *p, off_t obj_offset,
 	struct pack_window *w_curs = NULL;
 	off_t curpos = obj_offset;
 	void *data = NULL;
-	unsigned long size;
+	size_t size;
 	enum object_type type;
 	struct unpack_entry_stack_ent small_delta_stack[UNPACK_ENTRY_STACK_PREALLOC];
 	struct unpack_entry_stack_ent *delta_stack = small_delta_stack;
@@ -1943,8 +1947,10 @@ void *unpack_entry(struct repository *r, struct packed_git *p, off_t obj_offset,
 			      (uintmax_t)curpos, p->pack_name);
 			data = NULL;
 		} else {
+			unsigned long sz;
 			data = patch_delta(base, base_size, delta_data,
-					   delta_size, &size);
+					   delta_size, &sz);
+			size = sz;
 
 			/*
 			 * We could not apply the delta; warn the user, but
@@ -2929,7 +2935,7 @@ int packfile_read_object_stream(struct odb_read_stream **out,
 	struct odb_packed_read_stream *stream;
 	struct pack_window *window = NULL;
 	enum object_type in_pack_type;
-	unsigned long size;
+	size_t size;
 
 	in_pack_type = unpack_object_header(pack, &window, &offset, &size);
 	unuse_pack(&window);
diff --git a/packfile.h b/packfile.h
index 9b647da7dd..49d6bdecf6 100644
--- a/packfile.h
+++ b/packfile.h
@@ -456,9 +456,9 @@ off_t find_pack_entry_one(const struct object_id *oid, struct packed_git *);
 
 int is_pack_valid(struct packed_git *);
 void *unpack_entry(struct repository *r, struct packed_git *, off_t, enum object_type *, unsigned long *);
-unsigned long unpack_object_header_buffer(const unsigned char *buf, unsigned long len, enum object_type *type, unsigned long *sizep);
+unsigned long unpack_object_header_buffer(const unsigned char *buf, unsigned long len, enum object_type *type, size_t *sizep);
 unsigned long get_size_from_delta(struct packed_git *, struct pack_window **, off_t);
-int unpack_object_header(struct packed_git *, struct pack_window **, off_t *, unsigned long *);
+int unpack_object_header(struct packed_git *, struct pack_window **, off_t *, size_t *);
 off_t get_delta_base(struct packed_git *p, struct pack_window **w_curs,
 		     off_t *curpos, enum object_type type,
 		     off_t delta_obj_offset);