Well, as far as story content, nothing. But if we take a deeper look at the "Metropolis Mailbag" from that fateful issue back in 1968 (3rd and 4th images), we will find the following letter to the editor:
"Bulging biceps! I'm an amateur weight-lifter, and I think Superman can use some more muscularity. Couldn't he make a machine to duplicate a red sun's light so he could work out under its rays, and, not being invulnerable, build up his body by weight-lifting? It's my theory that if he did this, he could increase his size, strength, and muscularity."
- Carl Joy, Silver Creek, N.Y.
With the following reply from the editor:
"(Straining sinews! You're the first person who ever suggested that our super-specimen of breath-taking brawn wasn't muscular enough! Besides, when a guy can push planets around, how much more strength does he need?-Ed.)"
Who was this curious lad, questioning Superman's muscularity?!
My Dad, at 14 Years Old!
I have added some supporting pages from Superman 13 (2012) so you can see a bit more about the situation that our "super-specimen" is in these days...
A quick couple of quotes from Superman 13, Page 1:
"If it were possible to put the Earth on a scale--
--the planet would weigh in at 5,972 sextillion metric tons.
You have been essentially bench-pressing that for five days, Superman."
- Shay Veritas
"A scientific genius and consultant/confidante to Superman. Dr. Veritas created 'The Block', a facility near the center of the Earth and the only place where Superman can physically train and actually see results."
"A subterranean training facility run by Dr. Shay Veritas, where Superman can train physically."
(answers for the two questions above quoted from their respective CV wiki pages)
So, I ask you, when was the concept of "The Block" really invented? Who was the first to question Superman's muscularity?
My Dad, at 14 Years Old! :D