Backaches, neck aches, headaches and poor posture. I don't care how fit you are...it happens. Period.
I know one busty woman who doesn't suffer from this: cherri.
Conversely, I knew a woman about 7-8 years ago who told me she used to be a G-cup "boobs on a stick" back in high school, and subsequently got a reduction. (She wanted to be a C-cup, but her surgeon took her to a B-cup.) When I knew her, she was no longer the "stick" (the pendulum had swung the other way) and was still about a B-cup. Prior to the surgery, she complained about constant back pain. Years after the fact when I knew her, she still suffered from back pain. My guess is it was bad posture, learned early on from having had the big boobs at one time. Whether the learned behaviour was due to gravity or due to wanting to appear less busty (or both) makes me wonder how many busty women could alleviate back pain through a deliberate change in posture habits. (This might be helped/hindered by the (im)properly fitting bra, I'm sure.)
I will add that cherri can't jog/run because of her boobs. If she does treadmill time, it's a long but brisk walk. If she wants to kick it up any, she has to use the elliptical.
If I were to hazard a guess, I would say that busty women typically don't end up as Olympic swimmers, most likely due to the drag their boobs create in the water. And the busty triathlete would likely only fare reasonably well during the bike riding portion of the race.
One vote for "not so charmed".