Humor not withstanding, @Tim27, this is actually a very real issue. As one who works in a running store, one of the most critical pieces of sports equipment for women is a sports bra. Statistics show that 80% of women wear the wrong size sports bra, and that a proper fitting sports bra can reduce injury, fatigue and long-term effects on ligaments and tissue. There is no muscle in breast tissue.
More facts (as published by Brooks/Moving Comfort): 1 in 3 women are embarrassed by breast bounce when they exercise. 25% of women don't work out as hard as they could because of breast bounce, and more than 40% of women don't even wear a sports bra when exercising!
Also, a sports bra should be replaced every six months or so. A year at best ("no bra should celebrate a birthday"). Therefore, proper sizing and fit is important.