Question

why dwarft planets are small?

Answer 1

It's not like dwarf planets are small. The planets which are small are called dwarf planets. Dwarf means small.

Answer 2

a dwarf planet is, “a celestial body orbiting a star that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite because of being usually small"
even dwarf means small