Well, people do eat less, but also more. They eat less calories, but more mass and volume. It's real simple. Carbs weigh less than fat, people eat more carbs and less fat. But why are we fatter? Insulin. The sedentary argument is BS, it's not a cause, it's an effect. A formula 1 car is real quick and fast, but it's also extremely light. Imagine if it was twice the weight, it certainly wouldn't be as quick, ya? Same with humans, we just don't like to lug around an extra 50lbs for no reason, so when we have no choice in the matter, we tend to become sedentary.