Why do red giants expand?
When you heat a gas, it expands.
Hydrogen is being fused into helium inside stars. As the cores of the giant stars become so hot, that hydrogen fuses into helium much more quickly and then helium gets fused into heavier elements (lithium, silicon, oxygen, nitrogen etc.).Soon enough, the star runs out of hydrogen and begins fusing helium, which leads to the star expanding and leading it to its demise.