I've been wondering about this after hearing it several times before.
Exactly HOW dense are breasts? How dense can they possibly be? Most women have relatively small breasts compared to the rarer extremes, but even they complain about how their bust weighs so much that it gives them back-problems.
Unless breasts are secretly filled with lead or mercury, there's no way something that's not THAT big can be so heavy as to put strain on a woman's skeletal structure.
Besides, we call them "chest pillows" for a reason. Though cartelage has weight, they're supposed to be light, soft and smushy. How can something with the consistency of a fleshy pillow be so dense?