Welcome to Android Central! Is it turning into a Picture-in-Picture size? Does the app go back to the main YouTube screen, or does it exit to the homescreen with PiP still showing?
Minimizes? Or you get a small picture at the top of a page of other stuff? If that's the case, turn the phone to landscape mode and you should get a full-screen picture. (That's how the YT site works on Android now.)