Food intake increases, fat cells get bigger and produce more leptin as weight is gained. We're exposed to an ever-expanding cornucopia of tasty, convenient foods, freely available 24 hours a day. Obviously, genetics can't explain those changes. That's why elite cyclists and marathoners can put in massive miles without withering away. The good news, though, is that in overweight people above their natural weight, exercise doesn't seem to stimulate appetite as much, and they may lose weight.
Exercise also helps maintain any weight loss that does occur.
Also, regardless of whether you lose weight with exercise, you'll still be healthier if you work out. Even if it would help maintain weight, most of us probably wouldn't want to live on the human version of rat chow. But eating more of your food in forms that Mother Nature provides might help your set-point do its job. A recent study found that adults with higher fiber intakes gained less weight over a 10-year period. In an ideal world, it would be the weight you maintain without conscious effort.