Index: src/wp-includes/widgets/class-wp-widget-media-image.php
===================================================================
--- src/wp-includes/widgets/class-wp-widget-media-image.php	(revision 43669)
+++ src/wp-includes/widgets/class-wp-widget-media-image.php	(working copy)
@@ -67,7 +67,7 @@
 				'size'              => array(
 					'type'        => 'string',
 					'enum'        => array_merge( get_intermediate_image_sizes(), array( 'full', 'custom' ) ),
-					'default'     => 'medium',
+					'default'     => get_option( 'image_default_size', 'medium' ),
 					'description' => __( 'Size' ),
 				),
 				'width'             => array( // Via 'customWidth', only when size=custom; otherwise via 'width'.