diff --git a/delta.h b/delta.h
index eb5c6d2fdb..ab0279168c 100644
--- a/delta.h
+++ b/delta.h
@@ -28,7 +28,7 @@ void free_delta_index(struct delta_index *index);
  *
  * Given pointer must be what create_delta_index() returned, or NULL.
  */
-unsigned long sizeof_delta_index(struct delta_index *index);
+size_t sizeof_delta_index(struct delta_index *index);
 
 /*
  * create_delta: create a delta from given index for the given buffer
diff --git a/diff-delta.c b/diff-delta.c
index 43c339f010..9e1f9e6f95 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -125,9 +125,9 @@ struct unpacked_index_entry {
 };
 
 struct delta_index {
-	unsigned long memsize;
+	size_t memsize;
 	const void *src_buf;
-	unsigned long src_size;
+	size_t src_size;
 	unsigned int hash_mask;
 	struct index_entry *hash[FLEX_ARRAY];
 };
@@ -140,7 +140,7 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 	struct unpacked_index_entry *entry, **hash;
 	struct index_entry *packed_entry, **packed_hash;
 	void *mem;
-	unsigned long memsize;
+	size_t memsize;
 
 	if (!buf || !bufsize)
 		return NULL;
@@ -302,7 +302,7 @@ void free_delta_index(struct delta_index *index)
 	free(index);
 }
 
-unsigned long sizeof_delta_index(struct delta_index *index)
+size_t sizeof_delta_index(struct delta_index *index)
 {
 	if (index)
 		return index->memsize;