diff --git a/upb/message/internal/message.c b/upb/message/internal/message.c
index 5a1d44ea30554..9269570713898 100644
--- a/upb/message/internal/message.c
+++ b/upb/message/internal/message.c
@@ -49,7 +49,7 @@ const float kUpb_FltInfinity = UPB_INFINITY;
 const double kUpb_Infinity = UPB_INFINITY;
 const double kUpb_NaN = UPB_NAN;
 
-static size_t _upb_Message_SizeOfInternal(size_t count) {
+static size_t _upb_Message_SizeOfInternal(uint32_t count) {
   return UPB_SIZEOF_FLEX(upb_Message_Internal, aux_data, count);
 }
 
@@ -59,7 +59,7 @@ bool UPB_PRIVATE(_upb_Message_ReserveSlot)(struct upb_Message* msg,
   upb_Message_Internal* in = UPB_PRIVATE(_upb_Message_GetInternal)(msg);
   if (!in) {
     // No internal data, allocate from scratch.
-    size_t capacity = 4;
+    uint32_t capacity = 4;
     in = upb_Arena_Malloc(a, _upb_Message_SizeOfInternal(capacity));
     if (!in) return false;
     in->size = 0;
@@ -67,7 +67,7 @@ bool UPB_PRIVATE(_upb_Message_ReserveSlot)(struct upb_Message* msg,
     UPB_PRIVATE(_upb_Message_SetInternal)(msg, in);
   } else if (in->capacity == in->size) {
     // Internal data is too small, reallocate.
-    size_t new_capacity = upb_RoundUpToPowerOfTwo(in->size + 1);
+    uint32_t new_capacity = upb_RoundUpToPowerOfTwo(in->size + 1);
     in = upb_Arena_Realloc(a, in, _upb_Message_SizeOfInternal(in->capacity),
                            _upb_Message_SizeOfInternal(new_capacity));
     if (!in) return false;
diff --git a/upb/message/internal/message.h b/upb/message/internal/message.h
index e01a71609f2dc..dea517958d24c 100644
--- a/upb/message/internal/message.h
+++ b/upb/message/internal/message.h
@@ -88,8 +88,8 @@ upb_TaggedAuxPtr_MakeUnknownData(const upb_StringView* sv) {
 
 typedef struct upb_Message_Internal {
   // Total number of entries set in aux_data
-  size_t size;
-  size_t capacity;
+  uint32_t size;
+  uint32_t capacity;
   // Tagged pointers to upb_StringView or upb_Extension
   upb_TaggedAuxPtr aux_data[];
 } upb_Message_Internal;
@@ -200,7 +200,7 @@ UPB_INLINE bool UPB_PRIVATE(_upb_Message_NextExtensionReverse)(
   upb_Message_Internal* in = UPB_PRIVATE(_upb_Message_GetInternal)(msg);
   if (!in) return false;
   uintptr_t i = *iter;
-  size_t size = in->size;
+  uint32_t size = in->size;
   while (i < size) {
     upb_TaggedAuxPtr tagged_ptr = in->aux_data[size - 1 - i];
     i++;
diff --git a/upb/message/message.c b/upb/message/message.c
index c66c726b50b8a..7f480f860109c 100644
--- a/upb/message/message.c
+++ b/upb/message/message.c
@@ -87,7 +87,7 @@ void _upb_Message_DiscardUnknown_shallow(upb_Message* msg) {
   UPB_ASSERT(!upb_Message_IsFrozen(msg));
   upb_Message_Internal* in = UPB_PRIVATE(_upb_Message_GetInternal)(msg);
   if (!in) return;
-  size_t size = 0;
+  uint32_t size = 0;
   for (size_t i = 0; i < in->size; i++) {
     upb_TaggedAuxPtr tagged_ptr = in->aux_data[i];
     if (upb_TaggedAuxPtr_IsExtension(tagged_ptr)) {
@@ -168,7 +168,7 @@ void upb_Message_Freeze(upb_Message* msg, const upb_MiniTable* m) {
   // Extensions.
   upb_Message_Internal* in = UPB_PRIVATE(_upb_Message_GetInternal)(msg);
   // TODO: b/376969853 - use iterator API
-  size_t size = in ? in->size : 0;
+  uint32_t size = in ? in->size : 0;
   for (size_t i = 0; i < size; i++) {
     upb_TaggedAuxPtr tagged_ptr = in->aux_data[i];
     if (!upb_TaggedAuxPtr_IsExtension(tagged_ptr)) {