diff --git a/wp-admin/js/post-formats.js b/wp-admin/js/post-formats.js
index 10aab6174e..62c75f6642 100644
--- a/wp-admin/js/post-formats.js
+++ b/wp-admin/js/post-formats.js
@@ -111,15 +111,24 @@ window.wp = window.wp || {};
 			});
 
 			mediaPreview = function (attachment) {
-				var dimensions = '', url = attachment.url,
+				var w, h, dimensions = '', url = attachment.url,
 					mime = attachment.mime,
 					format = attachment.type;
 
 				if ( 'video' === format ) {
-					if ( attachment.width )
-						dimensions += ' width="' + attachment.width + '"';
-					if ( attachment.height )
-						dimensions += ' height="' + attachment.height + '"';
+					if ( attachment.width ) {
+						w = attachment.width;
+						if ( w > 600 )
+							w = 600;
+						dimensions += ' width="' + w + '"';
+					}
+
+					if ( attachment.height ) {
+						h = attachment.height;
+						if ( attachment.width && w < attachment.width )
+							h = Math.round( ( h * w ) / attachment.width );
+						dimensions += ' height="' + h + '"';
+					}
 				}
 
 				$('#' + format + '-preview').remove();
diff --git a/wp-content/themes/twentythirteen/functions.php b/wp-content/themes/twentythirteen/functions.php
index b9e588af47..03fffcc096 100644
--- a/wp-content/themes/twentythirteen/functions.php
+++ b/wp-content/themes/twentythirteen/functions.php
@@ -547,8 +547,11 @@ add_action( 'template_redirect', 'twentythirteen_content_width' );
  * @return array Filtered attribute list.
  */
 function twentythirteen_video_width( $atts ) {
-	if ( has_post_format( 'video' ) )
-		$atts['width'] = 724;
+	if ( ! is_admin() && has_post_format( 'video' ) ) {
+		$new_width = 724;
+		$atts['height'] = round( ( $atts['height'] * $new_width ) / $atts['width'] );
+		$atts['width'] = $new_width;
+	}
 
 	return $atts;
 }
diff --git a/wp-includes/media.php b/wp-includes/media.php
index aa52352055..e626f54817 100644
--- a/wp-includes/media.php
+++ b/wp-includes/media.php
@@ -956,12 +956,25 @@ function wp_video_shortcode( $attr ) {
 		'height' => 360,
 		'width' => empty( $content_width ) ? 640 : $content_width,
 	);
+
 	foreach ( $default_types as $type  )
 		$defaults_atts[$type] = '';
 
 	$atts = shortcode_atts( $defaults_atts, $attr, 'video' );
 	extract( $atts );
 
+	$w = $width;
+	$h = $height;
+	if ( is_admin() && $width > 600 )
+		$w = 600;
+	elseif ( ! is_admin() && $w > $defaults_atts['width'] )
+		$w = $defaults_atts['width'];
+
+	if ( $w < $width )
+		$height = round( ( $h * $w ) / $width );
+
+	$width = $w;
+
 	$primary = false;
 	if ( ! empty( $src ) ) {
 		$type = wp_check_filetype( $src );