By default videos in a Videos-only grid are displayed as square. Add the following JavaScript to your site to display them in a 16:9 format. This can be done manually using a custom JavaScript plugin. For further information see our documentation here.
$('.cff-album-item .cff-video').each(function(){
$(this).add( $(this).find('.cff-lightbox-link') ).css('height', $(this).innerWidth()/(16/9) );
});