Why is the sun bigger than the planets?

Normal stars and planets are created when gases, rock, and other materials clump together due to gravity.

A rocky planet would, first of all, have such strong gravity beyond a certain size that it would tend to cling to lighter gases like hydrogen and helium, eventually turning into a gas giant or star:

Once the mass reaches a certain point, approximately twice the mass of Jupiter and a factor of 11, some fusion reactions will begin, transforming the gas giant into a brown dwarf, or low-luminosity star.

It might grow big enough to initiate proper hydrogen fusion and turn into a typical star like our sun if it keeps absorbing gases from the surrounding cloud.

White dwarfs and neutron stars are the remnants of stars that have collapsed, and they are much smaller than other stars. A white dwarf has an incredible density—it is typically 60% the mass of our sun but about the size of the earth—while neutron stars are even denser.

The sun is bigger than the planets because it is a star, while the planets are celestial bodies that orbit the sun. Stars, like the sun, are massive, luminous spheres of plasma held together by gravity, whereas planets are much smaller and are formed from the material leftover after the formation of a star.