It's about hydrostatic equilibrium. Meaning that if a body has enough mass, it morphs into a spheroid shape. Like water droplets in if there was no air. Bodies that are big enough end up like that because of this effect (see the dwarf planets Ceres, Eiris, etc. They are all round, but smaller stuff just stays a potatoe forever, like mars' potatoe moons Phobos and Deimos).

Scroll down to check out Planetary geology.

https://en.wikipedia.org/wiki/Hydros...m#Astrophysics