The NASA page https://www.nasa.gov/multimedia/videogallery/index.html  is a good example.  If you view it on desktop you'll see their video is pretty tiny.  I start watching it and realise I'm squinting.  "Is this a video for ants!"  So I drag and resize that video a bit larger. 

For your second question,  that's really interesting idea but can't think of when I'd do that, except for video of course.  Or an image maybe, a gif even, but just as a novelty factor (by which I mean memes).