diff --git src/wp-includes/widgets/class-wp-widget-media-image.php src/wp-includes/widgets/class-wp-widget-media-image.php
index 00b605420a..d26b34fad0 100644
--- src/wp-includes/widgets/class-wp-widget-media-image.php
+++ src/wp-includes/widgets/class-wp-widget-media-image.php
@@ -67,7 +67,15 @@ class WP_Widget_Media_Image extends WP_Widget_Media {
 				'size'              => array(
 					'type'        => 'string',
 					'enum'        => array_merge( get_intermediate_image_sizes(), array( 'full', 'custom' ) ),
-					'default'     => 'medium',
+
+					/**
+					 * Allow changing the default value without affecting the saved option.
+					 *
+					 * @since 5.1.0
+					 *
+					 * @param string The value of the image_default_size option.
+					 */
+					'default'     => apply_filters( 'widget_media_image_default_size', get_option( 'image_default_size', 'medium' ) ),
 					'description' => __( 'Size' ),
 				),
 				'width'             => array( // Via 'customWidth', only when size=custom; otherwise via 'width'.