Things change with time. Pandaria was supposed to be just another zone in south Kalimdor, but instead Blizzard made it its own continent instead. The point is that it doesn't really matter how it was originally envisioned, things can still go bigger or smaller when they decide to implement it. IF --and that's a big 'if'-- added mid-TWW, undermine is certainly not going to be 'continent-sized', for example. At best, it'll be at best a single zone, but more likely just part of a zone.