The cup sizes behind these numbers are a, b, c and d. The difference in bust refers to your upper bust minus your lower bust. The allowable error is plus or minus1.25cm. If your bust size difference is 13cm, it is of course a B cup. The cup sizes behind these numbers are a, b, c and d. The difference in bust refers to your upper bust minus your lower bust. The allowable error is plus or minus1.25cm. If your bust size difference is 13cm, it is of course a B cup. The difference between lower bust and bust AA :7.5cm, A: 10cm, B: 12.5cm, C: 15cm, D: 17.5cm, E: 20cm, F: 22.5cm..