diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index e1678dc..131aaf3 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -462,6 +462,13 @@ void rb_gc_impl_set_params(void *objspace_ptr) { }
 
 static VALUE gc_verify_internal_consistency(VALUE self) { return Qnil; }
 
+#define MMTK_HEAP_COUNT 5
+#define MMTK_MAX_OBJ_SIZE 640
+
+static size_t heap_sizes[MMTK_HEAP_COUNT + 1] = {
+    40, 80, 160, 320, MMTK_MAX_OBJ_SIZE, 0
+};
+
 void
 rb_gc_impl_init(void)
 {
@@ -469,7 +476,7 @@ rb_gc_impl_init(void)
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("BASE_SLOT_SIZE")), SIZET2NUM(sizeof(VALUE) * 5));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RBASIC_SIZE")), SIZET2NUM(sizeof(struct RBasic)));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_OVERHEAD")), INT2NUM(0));
-    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVARGC_MAX_ALLOCATE_SIZE")), LONG2FIX(640));
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVARGC_MAX_ALLOCATE_SIZE")), LONG2FIX(MMTK_MAX_OBJ_SIZE));
     // Pretend we have 5 size pools
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("SIZE_POOL_COUNT")), LONG2FIX(5));
     OBJ_FREEZE(gc_constants);
@@ -485,10 +492,6 @@ rb_gc_impl_init(void)
     rb_define_singleton_method(rb_mGC, "verify_compaction_references", rb_f_notimplement, -1);
 }
 
-static size_t heap_sizes[6] = {
-    40, 80, 160, 320, 640, 0
-};
-
 size_t *
 rb_gc_impl_heap_sizes(void *objspace_ptr)
 {
@@ -609,8 +612,8 @@ rb_gc_impl_new_obj(void *objspace_ptr, void *cache_ptr, VALUE klass, VALUE flags
     struct objspace *objspace = objspace_ptr;
     struct MMTk_ractor_cache *ractor_cache = cache_ptr;
 
-    if (alloc_size > 640) rb_bug("too big");
-    for (int i = 0; i < 5; i++) {
+    if (alloc_size > MMTK_MAX_OBJ_SIZE) rb_bug("too big");
+    for (int i = 0; i < MMTK_HEAP_COUNT; i++) {
         if (alloc_size == heap_sizes[i]) break;
         if (alloc_size < heap_sizes[i]) {
             alloc_size = heap_sizes[i];
@@ -658,7 +661,7 @@ rb_gc_impl_heap_id_for_size(void *objspace_ptr, size_t size)
 bool
 rb_gc_impl_size_allocatable_p(size_t size)
 {
-    return size <= 640;
+    return size <= MMTK_MAX_OBJ_SIZE;
 }
 
 // Malloc