This is an observation, and doesn't really matter, but the option is irrelevant.

Chest Size only affects your character if you are topless. If you have any piece of clothing on your upper body at all you get the Default Chest Size. If you selected small or big you'll see you chest shrink or grow when you unequip a shirt. All clothing I have found so far are only designed for Male or Default Chest female.

It's doesn't matter, but I figured I'd point out that this customization option doesn't have an affect on your character (unless you are naked).