I think I remember reading somewhere that if you go into each contact individually and edit the photo, it'll somehow make them go full screen again. I was having this problem, went through all 76 of my contacts, and it seems to have done it.
Go to the contact, choose edit, edit the photo, just move it a little and then choose done.
Sounds like a pain in the butt, I know, but if it's really bugging you it might be worth a shot.
Maybe try it on one contact first and see if it works before doing them all. Hope that helps!
This happened from a backup from an iPhone 5s to iPhone 6 and I fixed it by EDITING the contact photo and just re-positioning them. This didn't work for a few of them so I then DELETED/REMOVED the photo for the contact, then re-added the same photo and all as well.