How to fix small videos on iPhone

Some videos I receive through text come out a decent size. I had already discovered that if I tapped on it it would fill my whole plus size iPhone screen. Some videos I receive through text though, come out about a half inch tall. Tapping on them increases the size but nowhere near filling the whole screen. We can’t figure out why some people receive the videos in full size, and some only get these tiny thumbnails where even tapping on it does not get a full sized screen. There is more to the problem/solution then tapping on the video image.

[Re-Titled by Moderator]