Why is Mercury a planet although some dwarf planets are bigger than it.

Question: Why is Mercury a planet although some dwarf planets are bigger than it.
- Keywords:
  mercury,
  planet,
  universe
Asked by oharbinson to Jimmy, Niall on 22 Mar 2013.
- James Holloway answered on 22 Mar 2013:
  
  Good question! It comes down to a technical definition.
  
  Mercury has eaten up all the other rocks, satellites and debris in its orbit, and has had its shape rounded off by gravity. Therefore its a planet.
  
  Other dwarf planets that are bigger than Mercury may have elliptical (not very circular) orbits, so have a much harder job of getting all the debris in their orbit, so are dwarf planets.

Question: Why is Mercury a planet although some dwarf planets are bigger than it.