Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 35918)
+++ src/wp-includes/media.php	(working copy)
@@ -1206,6 +1206,10 @@
  * @return string Converted content with 'srcset' and 'sizes' attributes added to images.
  */
 function wp_make_content_images_responsive( $content ) {
+	if ( is_feed() ) {
+		return $content;
+	}
+
 	if ( ! preg_match_all( '/<img [^>]+>/', $content, $matches ) ) {
 		return $content;
 	}