Superman's build should be thin and scrawny, because nothing is heavy enough to offer him a proper resistance workout.
Adam Rifkin stashed this in Superman!
From the Reddit comments:
Superman had a workout room in his Fortress of Solitude. Apparently it was equipped with a Red Sun light so that he would lose his powers and be able to work out like a normal man.
[And he would talk out loud to himself while doing so.] "Now for some Super Exercises!"
According to Noam Chomsky the world-renowned linguist, it takes an act of will to not talk to oneself because language is intertwined with cognition itself i.e. our thoughts are inherently linguistic and language itself is both an expression and state of the human mind.
Stands to reason that in the Fortress of Solitude, uninhibited by social norms, everyone would be expressing their own thoughts out loud.
Segment starts round about 18 minutes in: