This sounds like a description of the JetPack’s Photon service.
Photon is an image acceleration and modification service for
Jetpack-connected WordPress sites. Converted images are cached
automatically and served from the WordPress.com CDN. Images can be
cropped, resized, and filtered by using a simple API controlled by GET
query arguments. When Photon is enabled in Jetpack, images are updated
on the fly.
The Photon API includes the fit GET parameter:
http://i0.wp.com/example.com/wp-content/uploads/2014/04/abc.jpg?fit=600%2C400
where %2C is url encoded comma.
It uses the image_downsize filter to overwrite the image retrevial with:
// Generate Photon URL
$image = array(
jetpack_photon_url( $image_url, $photon_args ),
false,
false
);
where the width and height are set to false.