Skb Have you checked on a mobile to see if the video is still stretching out of the right hand side?
If you check in firefox in responsive view mode, it still looks like it is.
Because your adding these to a post they still need to be responsive if added at a certain size. I personally use Ohara YT embed on my sites, as it's responsive, and resizes the videos on the fly. You can even see them change size just by zooming the page. But here's a couple of screenshots captured while using Firfox's responsive view mode. One from your site, and one from one of my test sites.