Airplay not full-screen

Hello, my Apple friends. I have a Vizio TV, a 4K Apple TV and an iPhone XS Max (upgraded from my 7 Plus). When I used to stream video (sports, specifically) the video, which is full screen on my phone, is NOT full screen on my TV. This didn't happen with my 7 Plus. I've done everything the same, and have taken all the steps I know of on the TV, Apple TV and phone to try and figure this out. Alas, here I am, watching TV from 2002. (see the attached picture. Sorry it's so dark, but you can see a little bit of the TV frame).

Anyone have a suggestion or two?

You're the best!