Well, only very rarely does a mutation lead to obesity. Genetics is definitely a factor, though. Thyroid hormone, Cortisol, Leptin, etc. several ingredients effect how well we distribute energy, how efficiently we store/use it, our ability to feel full and resist overeating. Some people are blessed with the positive genetic attributes, others with negative ones. Those who start behind will struggle to maintain normal weight and fitness, while the lucky ones can manage with little or no effort. Of course, we are lazier and more inclined to eat junk... :S
