Index: wp-content/themes/twentyfifteen/css/editor-style.css
===================================================================
--- wp-content/themes/twentyfifteen/css/editor-style.css	(revision 54300)
+++ wp-content/themes/twentyfifteen/css/editor-style.css	(working copy)
@@ -268,7 +268,9 @@
 .mce-item-table td {
 	border: 1px solid #eaeaea;
 }
-
+table td{
+	font-size: inherit!important;
+}
 table a {
 	color: #333;
 }