diff --git a/src/imagew-internals.h b/src/imagew-internals.h
index 3031359..0e520ef 100644
--- a/src/imagew-internals.h
+++ b/src/imagew-internals.h
@@ -19,8 +19,8 @@
 typedef double iw_tmpsample;
 
 #ifdef IW_64BIT
-#define IW_DEFAULT_MAX_DIMENSION 1000000
-#define IW_DEFAULT_MAX_MALLOC 2000000000000
+#define IW_DEFAULT_MAX_DIMENSION 40000
+#define IW_DEFAULT_MAX_MALLOC 2000000000
 #else
 #define IW_DEFAULT_MAX_DIMENSION 40000 // Must be less than sqrt(2^31).
 #define IW_DEFAULT_MAX_MALLOC 2000000000