The second most important piece of womens running kit! (After shoes!)
The breasts are composed of mainly fatty tissue with the mammary glands and muscle. The muscle is deep so cannot help much with supporting the breasts. Coopers ligaments help hold the breasts up. They are thin bands interwoven into the breasts and are not very strong. If the breasts are not supported properly during excercise then repetitive bouncing can stretch the Coopers ligaments permenantly. Investment in a good quality sports bra helps to minimise stretch, sagging and pain.
Regardless of size all active women need to wear a sports bra. Unsupported breast movement can be as much as 4cm in A cups and upto 14cm in G cups.