I don't mind there being a large pool of players who can enter a game, but smaller groups are more stable. One of the problems with the table-top rp I found was that the game seemed to revolve around the more talkative, extroverted people in the group as they were always jumping in before quieter players had a chance to. In larger groups this meant that only they were driving the rp forward. Now in a smaller group setting they are more fun because the quieter people get a chance to input things. (This is when all the players are sat around in the same room.)
Online? In theory, it should be better but problems can occur if there is one or two people in the group who don't want a certain player in the rp. I'm not talking about when a player wants to join a group rp that is already been established, but one where people have been invited by the organizer and it's still a problem for that person. (It happens too often, to be honest.) Another problem can occur when people don't wait for one or more people in the role-play to respond. Be patient and wait! (It's worse when you're in the middle of writing your reply and they post theirs because your reply then looks out of place.)
I have mainly played 1x1, but it's not through choice, more because there aren't people willing to take on the roles of the characters I'd like for the role-play and although it can go well if both parties continue to contribute on a regular basis, it's annoying when you're waiting around for a day or more, just for one response. (I don't mind waiting like that if the other person is reliable, but I've had quite a few experiences with those that are fickle or are clearly active in other rps they have but seem to be deliberately ignoring yours for whatever reason. I suppose this can happen in group rps too, but I like to have a small-medium group of people to rp with so that I can rp with others while I am waiting for a response to another rp I have going, if that makes sense.)