Hi Esantee,
It could be possible that the photo isn't of a high enough quality therefore it can't appear full screen when a call comes in. Try setting another photo as the contact photo (if you haven't already) and see if that works. I know what you are talking about and it is nice to have the full screen contact photo appear, but keep in mind, in the fall when iOS 14 is released, apple will be removing the "full screen incoming call window" and slimming it down to a notification like form at the top. (Also note that when a FaceTime video call comes in, it will not show the contact photo full screen as your video is already full screened).
I hope that helps.
-Percival876