From what I understand, females have the potential to be bigger than males... but the practice of breeding them before they hit full size can reduce their ACTUAL size by quite a bit.
If you think about it, even in a very healthy young female of good weight, making eggs takes a lot of energy that isn't then being used to make a bigger female.
Whereas a male's contribution to breeding just isn't nearly so taxing on him, and he's got a lot more energy to spend on growing.