Hi - as i see it, the solution proposed above will only work until the next time the contacts are synced.
This worked for me;
My guess is that on your iPhone you sync your contacts with google as an "Exchange" account - or perhaps the "Google" account.
What fixed the problem for me;
Stop syncing with gmail the way you used to do, be either dropping the account on the iPhone or deselecting contacts from the account.
Next; add new account. Choose "Other",, choose "Add CardDAV-account, server: google.com, username: your gmail username, password: your gmail password, description: whatever you like.
Save account, and allow the contacts to sync.
Now, on your iPhone, add pictures to your contacts. These should stay as "fullscreen" even after future syncs.
Annoyingly this means, that if you edit your contacts through another device og through gmail - then the pictures will once again be "small". However - if you can live with primarily modifying contacts on your phone - then this should work.
Hope this solves your problem
Best regards
Theis