How to increase size of contact picture on iPhone

Contact photos assigned in your Mac's Address Book and synced to the iPhone will be thumbnails.

Photos assigned on the phone itself will be full screen.

You can edit the photo in your contacts on your phone to use a full-size photo.

Go to the contact with the small photo. Tap the "Edit" button at the top right.

Then tap the photo. From the list that pops up, choose the "Edit Photo" button.

Move and scale as you like and tap "Choose".

Finally, tap the "Done" button at the top right to save your change.