If you watch the movie, you’ll see his abdominal muscles as he’s breathing heavily. Not disagreeing with you, just pointing out that his physique is a bit more impressive than what a single photo might indicate. Definitely not the modern, roided-up action star physique. I wonder if he was even getting pumped before shooting, like modern actors do.

Some other examples are Christopher Reeve as Superman, Tom Selleck as Magnum PI, and Erik Estrada as Ponch in CHiPs. All were seen as strong, fit leading men in the late 70s and early 80s. Schwarzenegger is probably the main reason that roided-up physiques became so mainstream, along with WWF/WWE. Both exploded in popularity around the early 80s.