I think it just has to do with the pregnancy and every pregnancy is different. With my oldest daughter I went up to a DD from a C and with my youngest daughter I went from a C to a D.
I think it has to do with whether you're having a boy or girl too. I started out as a AA. With my boy, I went up to a full A, with my daughter I went up to a full B.
I agree, it varies from female to female. I went from an A to a D and now I'm an even smaller A than before I got pregnant.
It is different for every women, I went from a C to a D. They get bigger also when your milk comes in towards the end of the pregnancy.