Here's why:

* Alpha Centauri A is a yellow dwarf star, similar to our Sun, but it's slightly larger and more massive. It has a radius about 1.22 times that of the Sun and a mass about 1.1 times that of the Sun.

* Alpha Centauri B, the other star in the Alpha Centauri system, is smaller than the Sun. It's an orange dwarf with a radius of about 0.86 times that of the Sun and a mass about 0.9 times that of the Sun.

So, while the Alpha Centauri system has two stars, only one (Alpha Centauri A) is bigger than our Sun.