Here's why:

* The sun is a star, and stars fuse hydrogen into helium in their cores. This process releases energy, which is what makes the sun shine. However, as the sun burns through its hydrogen fuel, its core contracts and heats up.

* This increased heat causes the outer layers of the sun to expand.

* This expansion is a slow process, but over time, the sun will continue to grow larger.

The sun is not getting closer to Earth. While the sun's gravitational pull does affect Earth, the distance between the two is relatively stable. The Earth's orbit around the sun is also stable due to the balance of gravitational forces.

Important Note: The sun will eventually become a red giant, expanding significantly and engulfing Mercury, Venus, and potentially Earth. However, this is billions of years in the future.