Really, the old canard of one cup size less than your close female relatives is bunk.
You're right, if you already have on your slender frame a little bit of "boobage" (probably gynecomastia) you could end up with major league gazongas.
Or you can stay right where you are.
No one knows.
In my case, I have "persistent pubertal gynecomastia," and when I started on low dose 17 beta estradiol and an anti-androgen, my chest blossomed. Some much so, it scared me.
Hope that helps.