Picture doesn’t fill screen

Hi there. I have an issue where many of the channels on Xfinity Stream Beta App (on Samsung TV) don’t fill the screen. Most of them seem to be movie channels.

I cannot adjust 4:3/16:9/Fit to Screen settings as the Xfinity App locks this and the option is therefore ghosted in my Picture menu.

Its also nothing to do with watching SD/HD channels as the problem is the same on both from what I can tell.

Any ideas? Not much point having a big tv if the picture is boxed up in half the screen with black around it!