There's only one excuse for not wearing a sports bra when you run, and that's being male. Otherwise, no matter what size you are, you need to arm yourself against the irreversible effects of gravity as you run.
Why? Because once the ligaments around the breasts stretch, there's nothing you can do to shorten them again. Normal bras reduce breast movement by around 35 per cent, but a good sports bra achieves closer to 60 per cent. A- and B-cup sizes normally suit crop-top styles; larger sizes require moulded cups; and either way you should look for a bra which has been designed for high-impact activities.
