The exercise required to strengthen her spine and hold up the weight of breasts that size would make her body burn fat, like the fat that makes up breast tissue. Those would have to be silicone for this to make any sense.
She's really adorable! I like the little Himari trinket on the backpack. Part of me is tempted to ask if she should have a Jigglypuff motif somewhere (for those who don't know, Jigglypuff's Japanese name, Purin, means pudding).