Why will a star get bigger?

Answer 1

It will get bigger because it gets hotter.

In a star, gravity is trying to squash it together (make it smaller), however as the core of the star is squashed smaller it gets hotter and the nuclear reactions going on in the core begin to happen faster. This generates more heat and the pressure form this heat and light (energy) pushes the outer layers of the stare further away form the center of the star - the star inflates (gets bigger).

Stars get bigger when they leave the main sequence (run out of hydrogen to fuse in their cores) and their cores shrink and get hot enough to fuse helium and other heavier elements.

The other thing that happen as the star gets bigger and hotter is that it gets redder because although it is giving off more energy, the surface area over which this energy is being emitted has increased too and this mean each bit of the surface is no as hot as it was before it inflated.