There has to be more of the world because the map doesn't end at the ocean--beyond the eastern edge there has to be at least a little more land, for example, or it would show a coastline on the eastern edge. It looks to me, compared to maps of THIS world, that the land mass we see could be just part of an enormous continent and this part is just cut off from the rest by some rather daunting terrain.
I think the reason why that area hasn't been explored is because of passability issues and also because the lands we know have been so ravaged by war that no government has had the time or resources to devote to a large-scale exploration campaign; they're always too busy financing enormous armies or rebuilding ruined cities. The great explorers of Spain, Portugal, and so on had wealthy governments behind them who were secure enough politically that they could afford to go looking for trouble. As to why nobody has sailed west... well, why did it take Columbus so long? His ships, even without magic, were technologically advanced enough to make the crossing--the problem was a lack of preservatives or refrigeration. It's very difficult to get food when you're out in the middle of the ocean, and if you run out halfway there you'll be eating the crew by the time you make landfall on the other side--and that's assuming there are edibles at your destination, which you can't prove in advance. Columbus' big step was to just ignore this fact--to just pray he would make it. That's a big condition and not a lot of people will willingly take that risk.
