I can attest that big breasts + sports just suck. I don't really have back problems, but when you're jogging, dancing, or......horseback riding.....it kind of hurts when they're bouncing all over. Sports bras have gone a long way but really, it's still not enough.
That must suck. I understood that when they are too big they can cause back problems, etc. I never thought about how uncomfortable those things would probably feel yanking up and down while your running or something.