I just tried a Facebook app that automatically fetches pictures and birthday dates from Facebook and profile pictures. Thing about this app is that it “downgrades” any eventual full sized contact photos you are already using. I think it’s pretty much an undocumented, the fact that if you use a large photo within the iPhone to set a picture for a contact – it will fill the screen instead of just showing a small thumbnail to the upper right.
Here, lemme break it down. If you set pictures for your contacts using the Adress Book in your Mac, only small pictures will show on the iPhone. If you set pictures for your contacts using the iPhone – it will use large photos.
I don’t use the app mentioned above for this reason, nor do I use the Adress Book at all to set my pictures, I’ve done it all from within my iPhone to get the nice looking full screen photos. Below are screenshots from the iPhone showing the differences.