Something in new messaging interface CSS tries to squish the contents of the messages. It is especially obvious when you try to send somebody an image or a video. Video.js player in Moodle uses "fluid" layout where videos are automatically resized to 100% of their container width to ensure responsiveness (there is also max-width specified so they don't get too wide).

Probably the easiest way to see what's going on is to try to send somebody an image (I used https://www.ancestry.com/wiki/images/archive/a/a9/20100708215937!Example.jpg ).

1. Enable url-to-link and multimedia filter (urltolink first)
2. Open messaging window and try to send somebody a url to an image or a video file, for example https://www.ancestry.com/wiki/images/archive/a/a9/20100708215937!Example.jpg or http://techslides.com/demos/sample-videos/small.mp4
3. try to do the same but with the text in the same message