The bottom line is how much you eat. Documentaries like "Super Size Me" is full of false information and misconception. The amount of calories in a regular Big Mac Meal = 3/4 of how much I eat in a typical day. 1/2 if it's a big work out day. So all that documentary was, is a guy with an uncontrollable eating habit gaining weight.
Calories in calories out. There's no such thing as starvation mode. Nobody gets fatter because they eat too little.