The band measurement, (around the back, under the breasts), determines the number; the cup size relates to the difference between the band size and the full circumference around the fullest part of the breasts. Height is not a factor; 'east-west' distance is. 32C is very large breasts; 42C are considerably smaller to the eye, but comparable in volume size..
Cup sizes are different on every body and are a ratio with band size. I can wear a G38, H36 or I34 and they will all fit me, it is called sister sizes.