A good sports bra is as essential piece of kit as a good pair of trainers, whatever sports activity and whatever your cup size. A sports bra has proven to be twice as effective at minimising breast movement as a 'normal' bra. 44% of women who exercise regularly do not wear a sports bra.
All breasts, no matter what size, bounce during exercise. Your breasts can bounce up to 14 cm when unsupported during exercise. It's important to reduce this bounce, not just for your comfort, but to reduce strain on the breast ligaments that cause them to stretch, resulting in irreversible damage.