Filter amp_loop_image doesn’t work

Hi. Please, help me with my problem. I want to change image sizes by function, but it dosen’t work.

function ampforwp_change_image_size( $args ) {
    $args['image_size'] = 'thumbnail';
    return $args;
}
add_filter( 'amp_loop_image', 'ampforwp_change_image_size');

Where did I go wrong, or did I set the function incorrectly?