The YouTube channel GOLDEN ERA BOOKWORM. Had an article on Bobby Pandour.
He worked out nonstop with extremely light weights. Like basically walked around with 5lb dumbells all day long. Whenever he had a free moment he would do nonstop curls or laterals or raises. Like hundreds of them it was just a part of his everyday life. He also ate like shit.
These guys were muscular but not huge. The average man back then was more muscular, probably due in part to their diet not having processed foods, saturated fats and sugar in everything but they also had twice as much testosterone. So it wouldn't be much to add 10 or 15 lbs and look massive by comparison.