He was ill during filming and lost weight. As the movie wasn’t shot in sequence, the weight loss is quite noticeable in some scenes especially in his upper chest, arms and jawline.
Stay Hungry was shot in sequence and you could see him quite literally grow on screen as contest day got closer. Apparently the director thought he’d be too big for Sally Fields so he had to be 210lbs on day one of shooting. It’s a shame he didn’t pose next to Waller at the end of this movie or appear in Pumping Iron in or close to his 74 Mr O condition...