That is an amazing shot of his triceps.
Most haterz complain he had small triceps....

His delts are great....perfect proportion with the rest of his body.
No reason to believe he couldn't get them (and every other part) bigger if he wanted.
Arnold was not 100% devoted to BBing towards the end (Pumping Iron, Hercules in New York era) he would take long stretches off bodybuilding and get smaller to pursue acting, then come back and train for like a few months for a show.
If he was 100% bodybuilding (like Pros of today who spend their whole day shooting up, stuffing their faces, training and napping) he would be ginormously bigger.