If you added even 20 pounds of muscle to someone like Bale, you would have a totally different body...Do you have any idea what 20 extra pounds of muscle looks like?
Cap is supposed to be the epitome of human physical conditioning, on all levels. Unless I am mistaken, he supposed to be the stronges "normal" human in the Marvel universe, and the whole "super soldier serum" thing just has "steroids" written all over it, so either they get a guy who is a fairly big, muscular guy, or they pad the heck out of the suit...Of course, this would basically mean nothing to those who are not just casual comic book fans.
However...Marvel has already set us up for having a person take the serum and gain incredible abilities, without gaining any mass, such as when Blonsky took his first dose of the serum in TIH.
And to anyone who thinks that bodybuilding makes you slow and inflexible, you need to leave the 1950's behind, and get with us in the 21st century.
Pro wrestlers, footballers, hockey players, baseball players, MMA fighters, and even pro bodybuildiers are mostly large, muscular guys, and maintain better than average, and sometimes extrordinary flexibility. The whole, "I would bulk up, but I want to stay flexible" argument is touted by guys who simply are not atheletes.
Me, I would want someone who was built like Dwayne Johnson when he did "Walking Tall". Don't look at him now though, as he has intentionally lost quite a bit of mass.