Muzik: I'll give you the quick version, for any more go do some reading on social dynamics.
When groups are small they can manage them self, a group of 10 people can play together and moderate themselves. If one person is going out side the bounds/rules of the group. The 10 people can deal with the person and either have him change his behavior or remove him from the group.
Has groups get larger and larger people start becoming anonymous and start viewing the group as only a group and not individual people. As this effect gets larger the social structure of the group brakes down and people become more disruptive with out many social consequences. This is a very common social outcome. When more people joing a group it requires a much greater management and governance. How well does it work to have towns of 5000 people with out laws, police, firemen garbage men and other very formal rules of order and management roles.
This show up in game play in many ways and creates less enjoyment for the entire group.
As far as the business side it really is VERY simple, we had hit an artificial ceiling and the player count had reached it's maximum in one arena. It had reached a point where more people would degrade the play, and as people came more people left and more people choose not to become subscribers, the number of total players of AH had STOPPED growing.
As soon as we split the arenas, even with all the complaints and whines when we did, we started growing again the same week we split the arena.
HiTech