"If you look at the men's underwear, the styles are more practical and none of the models look overweight."
I want to see fit looking men, (if I was ever inclined to buy men's undies) and realistic looking women when buying my own bras.
I guess whether realistic looking male models are used, should be down to what male customers prefer to see, when shopping. I've got no skin in that game since I think the men in my life are more than capable of buying their own pants!