That's such a huge size difference though. That's like saying a watermelon is somewhere between the size of an ant and tylerchu. Well... you're not wrong...but damn if that isn't a huge size range.
Shortest version: Objects observed in the outer solar system are behaving(orbits and alignments) in a way that suggests that there is an object of sizeable mass out there causing this. It hasn't been discovered yet, just it's existence is one explanation for these observations.
Everyone please go home and come back next year.