Great observation! Damn he looks crazy in those pictures but Arnold still had the size. We will never know I guess. 
Sure, but Serge wasn't so tiny that he placed third, he did get second place ahead of Lou. Given that, look at the two most-musculars in those pics (one form the contest and one in the gym), he's just as cut in the gym but MUCH, MUCH bigger. I think, given that Arnold was down 20+ lbs in '75, Serge in that shape just
might have beaten him.