There is no need for Clark Kent to hide his physique. He is a "farm raised boy", and he is just a big guy.
Reeve portrayed this perfectly. In SMTM, when he is in Lois' apartment as Kent, trying to get Loois to go out for their date, Lois leaves the room. Clark, having made the decision to tell Lois he is Supes, takes off his glasses, and straightens his body. Reves gets about four inches taller and seems to gain about 20 pounds of muscle when he straightens up, and everytime I see it, I go, "Holy Crap, Reeve was awesome at hiding how tall and big he actually was!!" Reeve had a way of slouching and hunching his body to hide his true height and weight. That is part of his diguise.
Now Reeve was no Mr. Universe, but he did pack on considerable muscle for the part, even changing his jawline through muscle development. Routh was probably about the same size as Reeve when the movies where made, but his boyish, coupled with the claustorphobic cut of the costume, really destoyed the illusion of size.