Why does the Audi A2 have such a high fuel economy?

I've been considering purchasing an Audi A2 due to its advertised high fuel efficiency, but I'm not sure why it's able to achieve this. Can someone explain the specific technical features and factors that contribute to its high fuel economy?

The Audi A2's lightweight aluminum build and aerodynamic design greatly reduce its drag and consequently, its fuel consumption. From the engine to the body, every aspect of the car is optimized for efficiency.

In addition to its efficient exterior, the Audi A2 also has a start-stop system that shuts off the engine when the car is idle, reducing unnecessary fuel consumption. This feature can be found in Audi's other models as well.