Why are most stars in the Milky Way much smaller than our Sun?

A red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They constitute the vast majority of stars and have a mass of less than one-half of that of the Sun (down to about 0.075 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K.

An orange dwarf, also called a K V star, is a main sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main sequence stars of luminosity class V and yellow G-type main sequence stars of luminosity class V. They have masses of from 0.5 to 0.8 times the mass of the Sun and surface temperatures between 3,900 and 5,200 K.