Why is Ceres so big?

It may be that Ceres didn't originally come from the location of it's present orbit, or it may be that Ceres is simply the largest piece of debris left over after the solar system formed. Because of Jupiter's influence, the asteroid belt remains a belt - the debris doesn't have much of a chance to form into another planet; but if I were a bettin' man, I'd guess that Ceres was a chunk of rock that happened to be hanging around that area when Jupiter and the Sun formed.