diff --git a/lib/block-supports/behaviors.php b/lib/block-supports/behaviors.php
index 5f6c72e52199e3..a18700a5884105 100644
--- a/lib/block-supports/behaviors.php
+++ b/lib/block-supports/behaviors.php
@@ -97,9 +97,8 @@ function gutenberg_render_behaviors_support_lightbox( $block_content, $block ) {
 		$img_uploaded_srcset = wp_get_attachment_image_srcset( $block['attrs']['id'] );
 	} else {
 		$img_uploaded_src    = $z->get_attribute( 'src' );
-		$img_dimensions      = wp_getimagesize( $img_uploaded_src );
-		$img_width           = $img_dimensions[0];
-		$img_height          = $img_dimensions[1];
+		$img_width           = 'none';
+		$img_height          = 'none';
 		$img_uploaded_srcset = '';
 	}
 
diff --git a/packages/block-library/src/image/view-interactivity.js b/packages/block-library/src/image/view-interactivity.js
index 441810a9f2270b..e0fd68646964b4 100644
--- a/packages/block-library/src/image/view-interactivity.js
+++ b/packages/block-library/src/image/view-interactivity.js
@@ -202,8 +202,18 @@ store( {
 } );
 
 function setZoomStyles( imgDom, context, event ) {
-	let targetWidth = context.core.image.targetWidth;
-	let targetHeight = context.core.image.targetHeight;
+	// Typically, we use the image's full-sized dimensions. If those
+	// dimensions have not been set (i.e. an external image with only one size),
+	// the image's dimensions in the lightbox are the same
+	// as those of the image in the content.
+	let targetWidth =
+		context.core.image.targetWidth !== 'none'
+			? context.core.image.targetWidth
+			: event.target.nextElementSibling.naturalWidth;
+	let targetHeight =
+		context.core.image.targetHeight !== 'none'
+			? context.core.image.targetHeight
+			: event.target.nextElementSibling.naturalHeight;
 
 	// Since the lightbox image has `position:absolute`, it
 	// ignores its parent's padding, so we need to set padding here