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 enormous amounts of energy, which creates outward pressure.

* Gravity: The Sun's own gravity pulls inward, trying to collapse it.

* Balance: The outward pressure from fusion and the inward pull of gravity are currently in a delicate balance. This balance is what keeps the Sun stable.

However: The Sun is slowly running out of hydrogen fuel in its core. As fusion slows down, the outward pressure will decrease. Eventually, gravity will overcome the pressure, and the Sun will begin to contract. This contraction will heat the core even further, eventually igniting a new phase of fusion, where helium fuses into heavier elements. This is what will cause the Sun to become a red giant, much larger than its current size.

The expansion is a slow process: It takes billions of years for stars to evolve significantly. So, while the Sun is expanding, we won't notice the change in our lifetimes.