YouTube regularly transcodes all uploaded content into several different varieties of MP4 container. If you're on iOS, then all videos you watch on YouTube are MP4s.
I don't mean lightweight in a technical sense, I mean lightweight in terms of user experience. To view a gif (or gif converted to mp4, webm, whatever) I know it's silent (don't have to worry about muting my phone), I don't have to load the youtube app or the bloated youtube site. All I do at the most is tap to play.
