What causes a red giant to become so large?
When a star uses up all its hydrogen, helium is then fused into carbon.
A "main sequence" star like our sun, uses its vast supply of hydrogen and fuses it to create helium. The energy released from this fusion keeps the star from collapsing in on itself because its gravity is so great.
Eventually, the hydrogen will run out and all the star is left with is helium. It will begin to shrink and become more dense, temperature will increase and this new, hotter temperature and density will allow helium to begin fusing to form carbon. This new fusion will release massive amounts of energy causing the star to expand to many times its original size and a red giant is formed.