In the original show? Adam looks just as muscular to me. Paler, but very buff.
And a lot of subtle nuances in Christopher Reeve’s portrayal (and I’m using him as an example, because it’s the best) helped to make Kent appear to be a distinct character. Like parting his hair on the opposite side. And frankly, the point of Kent’s meek, nerdy persona was to make sure people didn’t pay attention to him. The surrounding characters’ impression of Kent as ignorable pretty much ensures they won’t even be able to conceive of him being heroic, let alone superheroic.
I always got the sense they were going for something like that in the original MotU, but they never pulled it off. In my opinion. Adam seems like an action figure that came out of the same mold.