Here's why:

* Nuclear Fusion: The sun's energy comes from nuclear fusion in its core, where hydrogen atoms fuse to form helium. This process releases a tremendous amount of energy, which makes the sun shine.

* Hydrogen Depletion: As the sun continues to fuse hydrogen, its core gradually becomes denser and hotter. This causes the fusion rate to increase, making the sun brighter and slightly larger over time.

* Red Giant Phase: Eventually, the sun will run out of hydrogen fuel in its core. At this point, it will start to fuse helium, causing it to expand dramatically into a red giant.

So, while the sun is not getting smaller, it is getting bigger, and in the distant future, it will become much larger than it is today.