Why is the sun bigger than the planets?
Answers
Explanation:
Normal stars and planets form by the clumping together of gases, rock, etc under the force of gravity.
First of all, beyond a certain size a rocky planet would have such strong gravity that it would tend to hang onto lighter gases like hydrogen and helium. It would then tend to become a gas giant or a star:
Beyond a certain mass - say about 12 times the mass of Jupiter and double the size - some fusion reactions will start and the gas giant become a brown dwarf - i.e. a star with low luminosity.
If it continues to accumulate gases from a surrounding cloud then it may become large enough to start proper hydrogen fusion and become a normal star like our sun.
There are stars which are much smaller, the remains of collapsed stars, namely white dwarfs and neutron stars. A white dwarf is incredibly dense, being typically 60%the mass of our sun, but about the size of the earth. Neutron stars are even more dense.