Why is the Audi A2 the most fuel-efficient vehicle in its class?

I've read that the Audi A2 has exceptional fuel efficiency compared to other vehicles in its class. Can anyone explain why this is the case and what makes the Audi A2 stand out in terms of fuel efficiency?

The Audi A2 is equipped with a lightweight aluminum body that significantly reduces its overall weight, leading to improved fuel efficiency. Additionally, the vehicle's aerodynamic design also plays a key role in reducing fuel consumption.

The Audi A2 comes with an advanced start-stop system that automatically shuts off the engine when the vehicle is idling, saving fuel consumption and reducing emissions. This feature sets it apart from other vehicles in its class.