Ive always felt that Reeve, and Routh and all the actors who have played Superman were not muscular enough. Reeve was more muscular yet even he had a bad case of lagging arms.
It my opinion that Routh looked emarassingly thin in the costume, his physique looked boyish, and he has effeminate hips. This could have easily been the fault of the costume design itself, but it did not look right on screen. Not to me anyway.
But why can he even have big muscles if there are no weights for him to lift? Why not? He is an Alien. We cannot apply human exercise physiology to an Alien.
He can simply be very muscular from the effects of the yellow sun, or even simply genetics.
Superman in every way is not just Super-powers-man, he should be Super Manly as well, and the muscular physique adds to such a look.
Simply my preference on the aesthetic. I think big muscles look good on Superman, and there is a reason why most comic artists depict him as very muscular. Same reason they draw him with a square Jaw, cleft chin and masculine facial features.
Im not saying we need him to be so big he should be obvious as Clark, the muscularity should not cross that line.
How big is THAT however where he does not look obvious as Clark? Well bigger than one might assume. The right clothing and posture can easily hide a great deal of muscularity, and can even make a man who is chiseled appear to be fat.
Grant Morrisons Clark is a perfect example of this. Chin down, back arched forward, and shoulders slumped, in business clothes he looks fat and dumpy. Nobody can tell it is muscle under the clothes. No definition is shown. Also very muscular men can even look like they have a gut if they push their stomach outward.
So long as he is not built like a Superheavyweight IFBB Pro, I dont think it would be that hard to convincingly (to the movie audience) conceal his physique to look like a fatty in a suit.