diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 107b2d1..311f74b 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -81,14 +81,13 @@
  * Size checking
  */
 
-#define ASSERT_SIZE(_thing, _size) ASSERT_STATIC (sizeof (_thing) == (_size))
-
-#define _DEFINE_SIZE_ASSERTION(_size) \
-  inline void _size_assertion (void) const { ASSERT_SIZE (*this, _size); }
+#define _DEFINE_SIZE_ASSERTION(_compare) \
+  inline void _size_assertion (void) const \
+  { ASSERT_STATIC ((sizeof (*this)) _compare); }
 
 
 #define DEFINE_SIZE_STATIC(size) \
-  _DEFINE_SIZE_ASSERTION (size); \
+  _DEFINE_SIZE_ASSERTION (== (size)); \
   static const unsigned int static_size = (size); \
   static const unsigned int min_size = (size)
 
@@ -97,14 +96,15 @@
 #define VAR0 (VAR+0)
 
 #define DEFINE_SIZE_MIN(size) \
+  _DEFINE_SIZE_ASSERTION (>= (size)); \
   static const unsigned int min_size = (size)
 
 #define DEFINE_SIZE_VAR(size, _var_type) \
-  _DEFINE_SIZE_ASSERTION ((size) + VAR0 * sizeof (_var_type)); \
+  _DEFINE_SIZE_ASSERTION (== (size) + VAR0 * sizeof (_var_type)); \
   static const unsigned int min_size = (size)
 
 #define DEFINE_SIZE_VAR2(size, _var_type1, _var_type2) \
-  _DEFINE_SIZE_ASSERTION ((size) + VAR0 * sizeof (_var_type1) + VAR0 * sizeof (_var_type2)); \
+  _DEFINE_SIZE_ASSERTION (== (size) + VAR0 * sizeof (_var_type1) + VAR0 * sizeof (_var_type2)); \
   static const unsigned int min_size = (size)