The cup size in a bra indicates the size of the breasts in relation to the rib cage, or how much bigger the breasts are than the rib cage. It is measured alphabetically, such as A, B, C, D, DD, E, F, and so on. The cup size is determined by the difference between the rib cage and the bust, and each cup letter represents an inch of difference between these measurements. For example, a difference of 1 inch corresponds to an A cup, 2 inches to a B cup, 3 inches to a C cup, and so on