Here's why:

* Red Giants are much larger than the Sun. They have expanded significantly as they age, increasing their surface area.

* Even though they are cooler than the Sun, the sheer size of their surface area means they emit a much larger total amount of light.

It's important to note that there is a wide range of red giants, with some being much brighter than others. The exact brightness of a red giant depends on its mass and stage of evolution.