I don't follow. Planet Nine is not expected to be another dwarf planet like Pluto. There are a handful of other dwarf planets known already, some bigger than Pluto, and it's expected that there are hundreds more yet to be discovered.
Some may be bigger than Earth, yet still classed as dwarf planets. Therefore, I think the definition is BS as it has nothing to do with the planet itself (such as its geography, size, etc) but only the bodies around it.
You say "may be bigger than Earth", but I'm unaware of any dwarf planets that are bigger than Earth. Are there any? Assuming there are none, that's rather the point: bodies that large would tend to clear out their local zone.
