During a star's main sequence, which is what the sun is in the middle of, there is a balancing act taking place between the crush of gravity pulling the star in, and the outward pressure of fusion pushing the star apart. As the star reaches the end of the main sequencing, the elements undergoing fusion change as all the hydrogen is converted to helium, and then the helium into carbon. This increases the outward pressure of the fusion, causing the star to swell