Superman's strength doesnt come from exercise either, yet actors cast work out to attain a certain build to play him [Hell, there'd be uproar if they didnt] so I dont see why the same wouldnt apply to WW.
I thought it would be pretty much a given that any actress cast would have to hit the gym to get into shape, and Carano's build there is that of an athlete, not a body-builder, and is definately
not too muscular in my opinion.
You want an example of a real overly muscled woman? Try this on for size:-
http://www.torahanaryu.com/images/chep9.jpeg
Not
that's too muscular. See the difference?