diff --git a/src/mem/largealloc.h b/src/mem/largealloc.h
index 14cbd53..cb347c0 100644
--- a/src/mem/largealloc.h
+++ b/src/mem/largealloc.h
@@ -366,8 +366,7 @@ namespace snmalloc
         p = memory_provider.template reserve<false>(large_class);
         if (p == nullptr)
           return nullptr;
-        memory_provider.template notify_using<zero_mem>(
-          p, bits::align_up(size, OS_PAGE_SIZE));
+        memory_provider.template notify_using<zero_mem>(p, rsize);
       }
       else
       {
@@ -390,8 +389,7 @@ namespace snmalloc
           // Passing zero_mem ensures the PAL provides zeroed pages if
           // required.
           memory_provider.template notify_using<zero_mem>(
-            pointer_offset(p, OS_PAGE_SIZE),
-            bits::align_up(size, OS_PAGE_SIZE) - OS_PAGE_SIZE);
+            pointer_offset(p, OS_PAGE_SIZE), rsize - OS_PAGE_SIZE);
         }
         else
         {
@@ -399,6 +397,8 @@ namespace snmalloc
           if constexpr (zero_mem == YesZero)
             memory_provider.template zero<true>(
               p, bits::align_up(size, OS_PAGE_SIZE));
+          else
+            UNUSED(size);
         }
       }
 
diff --git a/src/mem/sizeclass.h b/src/mem/sizeclass.h
index 76e774e..5b200d1 100644
--- a/src/mem/sizeclass.h
+++ b/src/mem/sizeclass.h
@@ -185,4 +185,13 @@ namespace snmalloc
 
     return ((alignment - 1) | (size - 1)) + 1;
   }
+
+  SNMALLOC_FAST_PATH static size_t round_size(size_t size)
+  {
+    if (size > size_to_sizeclass(NUM_SIZECLASSES - 1))
+    {
+      return bits::next_pow2(size);
+    }
+    return sizeclass_to_size(size_to_sizeclass(size));
+  }
 } // namespace snmalloc
diff --git a/src/override/malloc.cc b/src/override/malloc.cc
index 61aaf5e..cb1a2f6 100644
--- a/src/override/malloc.cc
+++ b/src/override/malloc.cc
@@ -86,7 +86,7 @@ extern "C"
 #endif
     size_t sz = Alloc::alloc_size(ptr);
     // Keep the current allocation if the given size is in the same sizeclass.
-    if (sz == sizeclass_to_size(size_to_sizeclass(size)))
+    if (sz == round_size(size))
       return ptr;
 
     void* p = SNMALLOC_NAME_MANGLE(malloc)(size);
diff --git a/src/test/func/malloc/malloc.cc b/src/test/func/malloc/malloc.cc
index 3ed01ed..d648d95 100644
--- a/src/test/func/malloc/malloc.cc
+++ b/src/test/func/malloc/malloc.cc
@@ -18,8 +18,13 @@ void check_result(size_t size, size_t align, void* p, int err, bool null)
   }
   else
   {
-    if (our_malloc_usable_size(p) < size)
+    auto asize = our_malloc_usable_size(p);
+    if (asize < size)
+    {
+      printf(
+        "Usable size is %zu, but required to be at least %zu.\n", asize, size);
       abort();
+    }
 
     if (static_cast<size_t>(reinterpret_cast<uintptr_t>(p) % align) != 0)
       abort();
@@ -30,7 +35,7 @@ void check_result(size_t size, size_t align, void* p, int err, bool null)
 
 void test_calloc(size_t nmemb, size_t size, int err, bool null)
 {
-  fprintf(stderr, "calloc(%d, %d)\n", (int)nmemb, (int)size);
+  fprintf(stderr, "calloc(%zu, %zu)\n", nmemb, size);
   errno = 0;
   void* p = our_calloc(nmemb, size);
 
@@ -47,7 +52,11 @@ void test_calloc(size_t nmemb, size_t size, int err, bool null)
 
 void test_realloc(void* p, size_t size, int err, bool null)
 {
-  fprintf(stderr, "realloc(%p(%d), %d)\n", p, int(size), (int)size);
+  size_t old_size = 0;
+  if (p != nullptr)
+    old_size = our_malloc_usable_size(p);
+
+  fprintf(stderr, "realloc(%p(%zu), %zu)\n", p, old_size, size);
   errno = 0;
   auto new_p = our_realloc(p, size);
   // Realloc failure case, deallocate original block
@@ -58,7 +67,7 @@ void test_realloc(void* p, size_t size, int err, bool null)
 
 void test_posix_memalign(size_t size, size_t align, int err, bool null)
 {
-  fprintf(stderr, "posix_memalign(&p, %d, %d)\n", (int)align, (int)size);
+  fprintf(stderr, "posix_memalign(&p, %zu, %zu)\n", align, size);
   void* p = nullptr;
   errno = our_posix_memalign(&p, align, size);
   check_result(size, align, p, err, null);
@@ -66,7 +75,7 @@ void test_posix_memalign(size_t size, size_t align, int err, bool null)
 
 void test_memalign(size_t size, size_t align, int err, bool null)
 {
-  fprintf(stderr, "memalign(%d, %d)\n", (int)align, (int)size);
+  fprintf(stderr, "memalign(%zu, %zu)\n", align, size);
   errno = 0;
   void* p = our_memalign(align, size);
   check_result(size, align, p, err, null);
@@ -81,6 +90,16 @@ int main(int argc, char** argv)
 
   constexpr int SUCCESS = 0;
 
+  test_realloc(our_malloc(64), 4194304, SUCCESS, false);
+
+  for (snmalloc::sizeclass_t sc = 0; sc < (SUPERSLAB_BITS + 4); sc++)
+  {
+    const size_t size = 1ULL << sc;
+    printf("malloc: %zu\n", size);
+    check_result(size, 1, our_malloc(size), SUCCESS, false);
+    check_result(size + 1, 1, our_malloc(size + 1), SUCCESS, false);
+  }
+
   test_calloc(0, 0, SUCCESS, false);
 
   for (snmalloc::sizeclass_t sc = 0; sc < NUM_SIZECLASSES; sc++)
@@ -97,11 +116,37 @@ int main(int argc, char** argv)
       test_calloc(n, 0, SUCCESS, false);
     }
     test_calloc(0, size, SUCCESS, false);
+  }
 
+  for (snmalloc::sizeclass_t sc = 0; sc < NUM_SIZECLASSES; sc++)
+  {
+    const size_t size = sizeclass_to_size(sc);
     test_realloc(our_malloc(size), size, SUCCESS, false);
     test_realloc(our_malloc(size), 0, SUCCESS, true);
     test_realloc(nullptr, size, SUCCESS, false);
     test_realloc(our_malloc(size), (size_t)-1, ENOMEM, true);
+    for (snmalloc::sizeclass_t sc2 = 0; sc2 < NUM_SIZECLASSES; sc2++)
+    {
+      const size_t size2 = sizeclass_to_size(sc2);
+      test_realloc(our_malloc(size), size2, SUCCESS, false);
+      test_realloc(our_malloc(size + 1), size2, SUCCESS, false);
+    }
+  }
+
+  for (snmalloc::sizeclass_t sc = 0; sc < (SUPERSLAB_BITS + 4); sc++)
+  {
+    const size_t size = 1ULL << sc;
+    test_realloc(our_malloc(size), size, SUCCESS, false);
+    test_realloc(our_malloc(size), 0, SUCCESS, true);
+    test_realloc(nullptr, size, SUCCESS, false);
+    test_realloc(our_malloc(size), (size_t)-1, ENOMEM, true);
+    for (snmalloc::sizeclass_t sc2 = 0; sc2 < (SUPERSLAB_BITS + 4); sc2++)
+    {
+      const size_t size2 = 1ULL << sc2;
+      printf("size1: %zu, size2:%zu\n", size, size2);
+      test_realloc(our_malloc(size), size2, SUCCESS, false);
+      test_realloc(our_malloc(size + 1), size2, SUCCESS, false);
+    }
   }
 
   test_posix_memalign(0, 0, EINVAL, true);
diff --git a/src/test/func/memory/memory.cc b/src/test/func/memory/memory.cc
index 8a0d122..cecb5fe 100644
--- a/src/test/func/memory/memory.cc
+++ b/src/test/func/memory/memory.cc
@@ -221,7 +221,7 @@ void test_external_pointer_large()
 
   for (size_t i = 0; i < count; i++)
   {
-    size_t b = snmalloc::bits::is64() ? 28 : 26;
+    size_t b = SUPERSLAB_BITS + 3;
     size_t rand = r.next() & ((1 << b) - 1);
     size_t size = (1 << 24) + rand;
     total_size += size;
diff --git a/src/test/func/sizeclass/sizeclass.cc b/src/test/func/sizeclass/sizeclass.cc
index 757d3a6..7d6eaf8 100644
--- a/src/test/func/sizeclass/sizeclass.cc
+++ b/src/test/func/sizeclass/sizeclass.cc
@@ -18,7 +18,7 @@ void test_align_size()
        size < snmalloc::sizeclass_to_size(snmalloc::NUM_SIZECLASSES - 1);
        size++)
   {
-    size_t rsize = snmalloc::sizeclass_to_size(size_to_sizeclass(size));
+    size_t rsize = snmalloc::round_size(size);
 
     if (rsize < size)
     {