Reeve looks bigger (and stronger) because he has a thicker neck than Routh's. Despite all the weight training, Routh still has a pencil neck look to him. I guesss it's genetics that no matter how much muscle you build, the wrists and ankles (or even the neck) will remain thin. Reeve, amazingly, has a thick neck even if he's not overly muscular, that he cuts an appropriately heroic profile especially when you view his head from the side.
One good example of this effect can be seen on the Batman cowls on Batman Begins and The Dark Knight. The BB cowl has a thicker neck, making Batman look more powerful (and panther-like) than in his TDK cowl which has a thin neck.