This is late Swan. From the 70s onward he slimmed him way down, away from the Wayne Boring template, which he indeed did continue for a long while. But from Murphy Anderson inking him onward, he was more like Gary Frank's physique for him recently.
I prefer that actually. I find that Superman much more relatable than the barrel-chested big-chinned guy, and his powers have nothing to do with his muscles(if Supergirl is any indication). But I grew up with the slimmer one, so I'm biased.
Besides, 50s and 60s Superman looks like he has emphysema.