Those bigger guys also have longer legs. If Cancellara was squished down, he'd be kneeing himself in the chest. Martin and Cancellara also look to have a flatter back. That may help with their aerodynamics. Levi's helmet isn't flat against his back. Levi also looks to be really compressing his diaphram. You need range of motion and the ability to breathe well to put out enough power. Like others have said, perhaps any power loss on Levi's end due to position is outweighed by his aerodynamics. That probably wouldn't be the case for the bigger guys.
Leipheimer's position is probably superior if you're testing coasting downhill. The question is whether you can sit like that and pedal hard on roads that aren't downhill.