and I find it very very hard to believe that one person would actually be able to pull something like that off.
As an example of MORPG made by one developer and his wife: http://www.eternal-lands.com/
You can easily google for Eternal Land's postmortem from Radu Privantu.
FYI, I was talking about an MMO, not an MO ;) I know there are MO's made by single developers.
When I say MMO I mean games like WoW, Guild Wars, EVE, etc. Basically games which are actually 'Massive'
I wouldn't put guildwars in the MMO category tbh, its so heavily instanced that it is essentially a normal multiplayer game for all practical purposes. (there can be lots of players in the towns but thats really just a fancy lobby for what is essentially a normal MORPG (highest number of players in a game in guildwars is 16 or 24(alliance battles involve 24 players but seem to be played as a series of 4vs4 games) which is hardly massive by any standard))