Superman's max weight is something like 10 Quintilian tons, but if his muscle mass were small would he still have that max weight?
What about this?
If I not mistaken, every cell in Kryptonian body is an energy converter, so less muscles = less body tissue = less cells to absorb sunlight = less sunlight to fuel his powers = weakened showings IMO.
A little clarification, please; what did you want to ask about?
Actually, considering he's not consistently lifting things which are heavy for him (since very little is heavy to him) he shouldn't have such large muscle mass at all.
He got buff when things like lifting trains and such caused strain. Not to mention, Clark was physically active growing up, long before his powers matured. Him being built isn't a surprise. And since his powers maintain his strength levels, he has no problem with losing muscle mass.
I don't think Superman's muscular physique make much of a difference to how strong he is. Superman is super strong because of gifted alien biochemistry. Him being muscular is just how he's drawn. Think of Supergirl. She's as strong as Superman, but is drawn like regular sex appeal female comic book character.