Stars have different sizes due to their mass and their individual stages during their evolutions, according to the Nova Celestia. Giant stars are particularly varied in size, as they have reached a stage wherein they start fusing helium — once the hydrogen runs out — into heavier elements in their core, causing them to grow larger.
As their mass increases, stars change in size, color, fusion rate and luminosity, according to Sun.org. Stars that are similar to the sun and have a solar mass of one are called G stars. They are characterized by a yellow color and a fusion rate that enables them to live for nearly ten billion years. Smaller stars have lower fusion rates because the temperature and pressure in their cores are lower compared to other stars.
The biggest stars are called O-type stars, which have a solar mass of 150 or more. Due to their high surface temperature, they show a blue color. There are incredibly high temperatures and pressures in their cores, causing them to burn hydrogen very quickly. Even though they have large amounts of hydrogen, it is all used up after a few million year,s which is quite short compared to other stars with longer lives.