Being a dwarf planet isn’t exclusively about size – actually, size is not explicitly mentioned at all in the definition of what a dwarf planet is!
In order to be classes as a dwarf planet, a planet needs to be orbiting around the Sun (as opposed to around another planet – this is why the Moon is not a dwarf planet, because it orbits around Earth).
It also needs to be spherical-ish in shape.
A final requirement is that it has not yet managed to clear its orbit of all other smaller objects – either by pulling them to its surface, or by getting them into a stable orbit around itself.
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