The breasts have a certain amount of nerve cells. When you first hit puberty and they're small, the nerves are tightly packed, hence making them hurt by the touch. As they grow, the nerves spread out, making it hurt less, if not at all.
every source i have says its false hun. there is a slight difference but since there is such a massive difference between woman's sensitivity and the difference between sizes are so small. the difference doesn't even matter. please don't c'aim science fact unless you have sources on hand to back it up.