Why do your boobs grow on birth control

Why do your boobs grow when you start birth control. I keep freaking myself out that I’m pregnant cus my boobs are still growing. I started birth control back in May. I used to be super flat chested like an A cup and now I think I’m getting closer to a C. I kinda liked my small boobs better. How long will they continue to grow? I’m 23 and I wish they would stop growing