#2 World of Warcraft
World of Warcraft never had any 'humble beginnings'. It was a game epic in scope since its inception, offering two vast continents: Kalimdor and the Eastern Kingdoms—the lands themselves based on the Warcraft series of real-time strategy games.
While we may never revisit Warcraft in the form of an RTS, we can certainly visit Azeroth in World of Warcraft, which has been expanded with the addition of Northrend and the Outlands—a whole different world—since its release. The most recent expansion, Cataclysm, even destroyed the lands of old to make them anew, and the upcoming Mists of Pandaria will add yet another land to visit.
It is difficult, if not impossible to compare the size of most other MMORPGs with Azeroth's world, as most of them are about as small as one of the larger areas in World of Warcraft.