To determine the cup size they need to measure the underbust and the bust. Take the bust minus the underbust. 0.5 inches is AA, 1 inches is A, 2 inches is B, 3 inches is C, 4 inches is D and so on. In my country a lot of girls had 30 to 32 inch underbust because their ribs is not that big. Plus they’re not overweight therefore they considered 30D or 32D to be a big bust size on a small frame. But the bigger the band size the bigger your breasts look.