It's a combination of two things.
Firstly, delts and traps, and arms are bigger today than they were in the 70s and early 80s due to improved drug regimens.
Secondly, when you're looking at side-chest comparision shots, the bodybuilders from that era did them slightly differently, puffing their chests out whereas todays physiques concentrate on showing the arms more, and contracting the chests which draws them in.