@Griatch
Mushes are more like Puzzle Pirates or URU; technically MMOs but not what most people think of when they talk about the genre. There are few "zones", and character ability does not unlock new areas, or at least no where at the rate that a game like Warcraft does.
Muds are the dungeon crawl that Lord Bishop turned into the MMO; conflict is programmed, automated, so a larger set of zones is critical. I can't speak for the RPI or cross-breeds where challenges come from mini games like crafting as much as mobs.
I wouldn't be surprised to find that the more the game relies on social interaction, codified or otherwise, the smaller the grid will be.
I have run games with a moderate to smaller player base with a hundred rooms, knowing that most of the rooms would barely be used, because it set the tone and pressed home the setting. This happened slowly, over time, by the players' need.
I think pushing the wrong sized grid on the players can easily backfire, and stresses the importance of an aware build staff.