Why is the fuel economy of the Audi A2 better than other cars?

I've been researching fuel efficient cars and came across the Audi A2. Can anyone explain why it has better fuel economy compared to other cars in its class?

The Audi A2 utilizes lightweight aluminum in its construction, resulting in a more efficient use of energy and ultimately, better fuel economy. In addition, the car comes equipped with start/stop technology, which shuts off the engine when the vehicle is idle, further reducing fuel consumption and emissions.