diff -r 6b46c3deea2c Objects/dictobject.c
--- a/Objects/dictobject.c	Tue Oct 25 11:51:54 2016 -0700
+++ b/Objects/dictobject.c	Wed Oct 26 04:03:13 2016 +0900
@@ -2403,15 +2403,17 @@
              */
             override = 1;
         /* Do one big resize at the start, rather than
          * incrementally resizing as we insert new items.  Expect
          * that there will be no (or few) overlapping keys.
          */
-        if (mp->ma_keys->dk_usable * 3 < other->ma_used * 2)
-            if (dictresize(mp, (mp->ma_used + other->ma_used)*2) != 0)
+        if (mp->ma_keys->dk_usable < other->ma_used) {
+            if (dictresize(mp, ESTIMATE_SIZE(mp->ma_used + other->ma_used))) {
                return -1;
+            }
+        }
         ep0 = DK_ENTRIES(other->ma_keys);
         for (i = 0, n = other->ma_keys->dk_nentries; i < n; i++) {
             PyObject *key, *value;
             Py_hash_t hash;
             entry = &ep0[i];
             key = entry->me_key;