why does the screen on my new iphone 12 pro keep enlarging

Could it be that you are using finger gestures to enlarge the screen. Using the Zoom gestures from Accessibility? Not seeing what you are doing, it is hard to say.

The same thing is happening to mine. I have to shut it down to correct. Totally missing my 7. Most of the changes in this model have added up to a worse user experience for me, but the enlarging screen is really annoying me.