Nutrition. Your body will prioritize utilization of nutrients when it’s in a deficiency. When it’s not, it can use whatever it has on everything. There’s obviously a tipping point where the body has to compensate for too much weight (and the stress it puts on organs and muscle, etc.), but since you haven’t really specified what “huge” is, it’s hard to say. Also, I’d venture that the girls you’re seeing get enough animal protein and the subsequent collagen.