Overwrite add_image_size defined by theme

Change the action to ‘init’:

class My_Plugin {
    public function __construct() {
        add_action( 'init', array( $this, 'overwrite_wptheme_settings' ) );
    }

    public function overwrite_wptheme_settings() {
        add_image_size( 'tie-small', 70, 70, false );
    }
}