Hmm, I just don't see a difference there. Nowhere in NV was the game world ever closed off in any of my games except for the Fort I guess and that was just the way the developters wanted it. The reason for the game ending was because Obsidian felt that fans really enjoyed the ending slides from previous games and those wouldn't work with a game that lets you continue after the end. As for how large the world is, I don't see how that makes any difference. Actually, with a larger world I would think that more dynamic events could be done.
Actually, it's pretty much the opposite. In a larger world (I'm talking about the number of characters, unique locations, etc.), there's more chances to break questlines. And since
Spoiler all the guilds have some kind of involvement in the main quest
, any actions that make major changes to guilds would most likely break that questline. Now, if the game had to be built around allowing every questline to remain intact after an almost infinite number of possible actions that a player could take, then the larger the game, the more impossible that task gets.