The piece of material attaching the cups should fit against your body. If it doesn't, you need a larger cup size.

A smaller band size also means a smaller cup size. Similarly, a larger band size means a larger cup size. This means that a bra sized as a 36A will have a larger cup size than a 34A, even though they are both A cups. Keep this in mind if you move up or down a band size.

You should measure yourself for a new bra after gaining or losing weight. If your weight has not changed, you should measure yourself once a year. 

When fitting yourself for a new bra, be sure to completely lengthen the straps and to use the loosest hook setting. This will enable you to tighten your bra as needed when it stretches with wear.