I vaguely remember one of those women (I forget which one) who did those giant tits lesbian videos doing an interview once where she explained that she didn't have backpain because she regularly stretched, jogged, and did a lot of exercise that helped her learn how to support and compensate for the movement of her breasts. She pointed out that most women don't exercise enough and she knew women with breasts much smaller than hers who complained about backpain because of this. It makes sense, too, since a lot of people feel bad because they're not very active and they put off on exercising because they're afraid it'll make them feel worse. I think this applies to both men and women, too.
