Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 47435)
+++ src/wp-includes/media.php	(working copy)
@@ -1786,7 +1786,7 @@
 	$style = '';
 
 	if ( $caption_width ) {
-		$style = 'style="width: ' . (int) $caption_width . 'px" ';
+		$style = 'style="max-width: ' . (int) $caption_width . 'px" ';
 	}
 
 	if ( $html5 ) {