I think this has nothing to do with less budget or whatnot.Its simply because ach world feels much bigger and full of stuff, so while a world took betwenn 1 and 2 hours to finish playing normally in KH II, I think in KH III they can go up until double, because they feel more full and bigger.
Plus, he said that there is fewer Disney wrolds, not fewer worlds.And since this is the last entry in the Seeker Of Darkness Saga, we could expect more unique worlds from the KH universe to show, such as Traverse Town, Keyblade Graveyard and Castle Oblivion/Land of Departure, and so on.
And who knows, we might see for the first time a world fully dedicated to a FF game!(Please be Spira, pleaseeee)