Why is the Audi A2 considered the most efficient car in its category?

I have heard that the Audi A2 is a highly efficient car, but I am not sure why. Can someone explain the specific features and technologies that make it so efficient?

The Audi A2 boasts an innovative aluminum frame design, which greatly reduces its weight and improves fuel efficiency. Definitely one of its standout features.

I've found that the A2's efficient aerodynamics, helped by its inclined C-pillars and rear spoiler, make a big difference in reducing fuel consumption at high speeds.