It's common knowledge among transwomen that (as a rule of thumb) your eventual cup size will be about one size less than that of your female relatives. But what if there's a rather large difference between the women in your family who have had children vs. the old spinsters?

I can't say for sure, but my guess is that one should look at the size of female family members before they've had children.

Wide shoulders also seems to diminish what size breasts look as compared to what they actually are. Many women also have wide shoulders (big boned) and they're breasts will tend to appear a size below their actual size. So 'Y's generally will seem to be one or two below on top of the cup size. (i.e. D > B or A as far as appearance is concerned.) More or less.