I have D's and I have to say that they are both a curse and a blessing. But, mostly a blessing. It does hurt to run but only if your running really fast, cause they pound againest your chest, but, if you wear a bra it doesn't happen as much and it controls it well. But, it kinda of feels like you have a wieght on your chest, which, is kinda annoying because they make you tired quickly. A treadmill is just like running anywhere, so, it doesn't really matter if you run on a treadmill or not. Swimming actually feels better because fat floats and since boobs are 90% fat, it feels great. Its the only time I don't feel wieghted down. Wieght lifting feels really good. It actually doesn't matter if your breasts are big or not.The more muscles you have the less it hurts when you wieght lift.the only for of exercising that hurts is running pretty much, everything else is ok really. Just as long as it doesn't take to much endurance it's ok. Dancing it alright too because it's so fun you forget about the wieght. I think having big breasts makes you get runners high quicker though, and that's kind of a bonus.
Hope that helps answer your question.