The point made in the book Why We Get Fat, is that obese people can't exercise more due to a problem with their fat regulation system. It's not that they don't want to exercise; it's that they physically cannot. A typical high-carbohydrate diet causes insulin to flood their bodies telling their fat cells to vacuum up blood sugar and make it inaccessible. The end result is that there's little to no fuel left available to the muscles. Muscles can't perform without an energy source.
The least invasive solution? Carbohydrate restriction to control insulin levels which opens up the possibility to again utilize fat normally (along with substantially-limited carbohydrates) for energy. Athletes aren't thin because they exercise. They exercise because they are thin.