Question

why do big stars explode whereas sun sized stars swell up and wither away

Answer 1

the big stars explode due to low hydrogen gas and sun gets wider because it still have too much hydrogen and scientists have discovered that sun will explode near 3000

Answer 2

the stars such as our sun is not so big to explode as it has not the mass required, and near it's end it will just swell up and become a red or white dwarf star. In the explosion of stars which are 25 times bigger than our sun, the time when the hydrogen is finished, the mass of the star decreases, then the angular momentum is not so strong to stop the gravity. The core shrinks, due to the extreme gravity and the elements of the mantle and core (include neutrinos) burst up shattering the crust to smithereens in the form of a huge wave of plasmic gamma, alpha and beta particles, we call that event as a supernova