Index: src/wp-content/themes/twentyseventeen/functions.php
===================================================================
--- src/wp-content/themes/twentyseventeen/functions.php	(revision 57859)
+++ src/wp-content/themes/twentyseventeen/functions.php	(working copy)
@@ -580,6 +580,7 @@
  * Filters the `sizes` value in the header image markup.
  *
  * @since Twenty Seventeen 1.0
+ * @since Twenty Seventeen 3.7 Added larger image size for small screens.
  *
  * @param string $html   The HTML image tag markup being filtered.
  * @param object $header The custom header object returned by 'get_custom_header()'.
@@ -588,7 +589,7 @@
  */
 function twentyseventeen_header_image_tag( $html, $header, $attr ) {
 	if ( isset( $attr['sizes'] ) ) {
-		$html = str_replace( $attr['sizes'], '100vw', $html );
+		$html = str_replace( $attr['sizes'], '(max-width: 767px) 200vw, 100vw', $html );
 	}
 	return $html;
 }