Bra sizes that are the same in breast volume are often referred to as “sister sizes”. The table above shows a range of bra sizes. The grey cells demonstrate how the sizes that diagonally adjacent from the top right to the bottom left are the same in breast volume across different circumferences i.e. the same volume of breast tissue spread across different size rib cages.
Sisters sizes demonstrate that if your bra cups fit your breasts perfectly but you need a different band size (because it is too tight or too loose), you will need to change the cup size as well to achieve the came fit in the cup. For example, if a woman who normally wears a 36D finds that the band is too loose but the cups fit perfectly, she will need to try a 34D to gain the same cup volume.