We have two X1 DVR’s, and both are exhibiting the same behavior, starting only a few weeks ago:
When the phone rings, the on-screen caller ID displays zoomed in. When caller ID disappears, the entire picture zooms in. I can reset it in the Xfinity menu, but it shouldn’t happen in the first place. The screen resolution setting matches the TV’s we’re using (4:3, 480i). We’ve tried warm and cold boots on the boxes, but nothing helps.
Have you tried changing the settings under caller ID, or phone settings. It sounds to me like there’s a conflict between how caller ID is set to appear, how your X1 Display is set to appear & how your TV settings are set. I would guess it’s a conflict with the first two & not your TV settings. I’m not with Comcast. Just another customer like yourself.
On my X1 boxes there are no options for caller ID other than On or Off. I did, however, find a setting which automatically defaults to the best available option for video playback. Since I have only old analog sets, I turned that option off. I'll post an update if this works, or doesn't.