As a former soldier, back pain is No myth. Try strapping about 6 pounds of gear around your Waist, walk around All day, then take it off, only to do it again for several straight days. There Will be lower back issues. Now, imagine that same scenario, only This time, you have 50-80 pounds of gear on your back as well, walking across terrain with hills, valleys, rivers, forests, marshes, and others to slow you down. After a couple of days, even the fittest of Men have small back pains. I know because I've treated them. Men are built to be load bearers and it Still gets to them.
Now, imagine being a female with large breasts. 36 DDD breasts can weigh nearly 1 pound per breast, which they can't remove, which shifts weight on the body depending on which side a woman leans or lies on her back. Gravity can make them seem heavier if they lean forward significantly, putting Tremendous strain on the back. Carrying those things on your chest every day, having to move them when you bathe or get dressed adds to the misery. Now imagine being a larger bra size. The weight naturally increases, more mass, more weight. That's why a number of large breasted women opt for reduction, taking some strain from their backs. Which is why I Commend ANY female with big breasts who don't opt for surgery. They generally bear the discomfort and don't gripe about it much.