Index: wp-content/themes/twentythirteen/content-gallery.php
===================================================================
--- wp-content/themes/twentythirteen/content-gallery.php	(revision 24022)
+++ wp-content/themes/twentythirteen/content-gallery.php	(working copy)
@@ -24,7 +24,7 @@
 			<?php the_content( __( 'Continue reading <span class="meta-nav">&rarr;</span>', 'twentythirteen' ) ); ?>
 			<?php wp_link_pages( array( 'before' => '<div class="page-links"><span class="page-links-title">' . __( 'Pages:', 'twentythirteen' ) . '</span>', 'after' => '</div>', 'link_before' => '<span>', 'link_after' => '</span>' ) ); ?>
 		<?php else : ?>
-			<?php twentythirteen_featured_gallery(); ?>
+			<?php the_post_format_gallery(); ?>
 		<?php endif; // is_single() ?>
 	</div><!-- .entry-content -->
 
Index: wp-content/themes/twentythirteen/functions.php
===================================================================
--- wp-content/themes/twentythirteen/functions.php	(revision 24022)
+++ wp-content/themes/twentythirteen/functions.php	(working copy)
@@ -445,39 +445,21 @@
 	return ( $has_url ) ? $has_url : apply_filters( 'the_permalink', get_permalink() );
 }
 
-if ( ! function_exists( 'twentythirteen_featured_gallery' ) ) :
-/**
- * Displays first gallery from post content. Changes image size from thumbnail
- * to large, to display a larger first image.
- *
- * @since Twenty Thirteen 1.0
- *
- * @return void
- */
-function twentythirteen_featured_gallery() {
-	$pattern = get_shortcode_regex();
-
-	if ( preg_match( "/$pattern/s", get_the_content(), $match ) && 'gallery' == $match[2] ) {
-		add_filter( 'shortcode_atts_gallery', 'twentythirteen_gallery_atts' );
-		echo do_shortcode_tag( $match );
-	}
-}
-endif;
-
 /**
  * Sets the image size in featured galleries to large.
  *
- * @see twentythirteen_featured_gallery()
- *
  * @since Twenty Thirteen 1.0
  *
  * @param array $atts Combined and filtered attribute list.
  * @return array
  */
 function twentythirteen_gallery_atts( $atts ) {
-	$atts['size'] = 'large';
+	if ( has_post_format( 'gallery' ) && ! is_single() )
+		$atts['size'] = 'large';
+
 	return $atts;
 }
+add_filter( 'shortcode_atts_gallery', 'twentythirteen_gallery_atts' );
 
 /**
  * Extends the default WordPress body class to denote: