So the breast cut serves no purpose other than making the armor look sexier and more feminine. Real breastplates provide plenty of room and besides, women need to bind their chests in order to fight comfortably (you don't want them to bounce) and therefore, armored shaped like a bra does nothing beneficial for you. It just looks sexy, which is fine for ceremonial armor that you'd never fight in, but that isn't the point of this sub.

Furthermore it's actually a disadvantage in a fight. Armor is shaped the way it is to deflect force away from you. Regardless of if a blow can pierce your armor, simply the fact you're getting hit hurts you and tires you out, so armor is designed with a curve to make force deflect away fro m you not into you. Breast armor (like the one featured in this drawing) causes force to be pushed toward you into your chest, resulting in you experiencing more trauma simply for the design. No one who actually fights in life or death battles would ever choose it over a traditionally non sexualized piece.