I'd suggest cutting off the upper arm completely and replacing everything from the shoulder to the elbow with a thinner, more cylindrical upper arm. you'll want something with stronger ball joints though... perhaps something from a revoltech figure. You may have to then alter the forearm a bit to be consistent. depends how much remodeling you're willing to do.
I think that would give a more feminine look.

Making the sides of the 'rib' area a bit more concave would really help too, if that's possible.