Geauga Lake was, at some point, much larger than Animal Kingdom. Since it has shrunk by some 99.9 % or more since then (there's still a water slide or something operating there, right?), Animal Kingdom has taken its place. Then again, that is mostly due to the enormous safari area. Next on the list, I don't know, but King's Island and Great Adventure seem to be worthy contenders.
Do anybody know if some of the Chinese theme parks could be up there? Any place that operates an amusement park and a safari-style zoo combined will easily eat up a lot of land, like Animal Kingdom (for instance, Norway's largest amusement park is also a zoo, it's thrice the size of TusenFryd). There appears to be quite a few places like that, so I doubt Animal Kingdom is very far removed from the next park on the list.