It seems there is confusion on your part in regards to sister sizes. The sister sizes all hold the same volume, which is what matters when it comes to bra cups. The depth and width of the cup tend to be close in measurement with close sister sizes (think 38GG/40G/42FF), but the further out you go, the larger the difference in cup shape (think 28F/34D/40A). This is because someone with a 28" ribcage is has a smaller shape and more projected breasts than someone with a 40" ribcage, and is why it's recommended to go up or down only one sister size in bras, otherwise fit can be greatly affected. (There are some people with bodies shaped in a way that going up several sister sizes can offer the best fit, but this is not a common occurance)
If you try on a 30F, and the cups hold all your breast tissue, but the band is difficult to close, going up a sister size, to 32E, would usually make it so the bra fits perfectly, or near perfect, now that both the cups hold all your breast tissue and the band can be closed with ease.
Here is an article by A Sophisticated Pair that has this handy video: