By default videos in a Videos-only grid are displayed as square. Add this snippet to the plugin’s Custom JavaScript section (Customize > Misc > Custom JavaScript) to display them in a 16:9 format.
$('.cff-album-item .cff-video').each(function(){
$(this).add( $(this).find('.cff-lightbox-link') ).css('height', $(this).innerWidth()/(16/9) );
});