Does not fit very well with greyhawk (or any other official campaign world i am aware of)I myself like to imagine that Athas (Dark Sun) is the future of Oerth (Greyhawk).
They feel closer together in tone to me.
spoiler, only read if you are not playing as a player in DS campaign:
Halflings where the absolute predominant race back in Athas past. On Athas they are basically the source of all life