diff --git a/src/include/utils/memutils.h b/src/include/utils/memutils.h
index 5ee533b30d..7c1202478e 100644
--- a/src/include/utils/memutils.h
+++ b/src/include/utils/memutils.h
@@ -30,7 +30,9 @@
  *		be summarily denied.
  *
  * XXX This is deliberately chosen to correspond to the limiting size
- * of varlena objects under TOAST.	See VARATT_MASK_SIZE in postgres.h.
+ * of varlena objects under TOAST.	See VARSIZE_4B() and related macros
+ * in postgres.h.  Many datatypes assume that any allocatable size can
+ * be represented in a varlena header.
  *
  * XXX Also, various places in aset.c assume they can compute twice an
  * allocation's size without overflow, so beware of raising this.