It's more about the community than the alliance. A good community will keep players, a bad one will loose players. There can be micros with good communities and active ones, however they aren't nearly as common as micros that cause people to get bored and quit easily. There are large alliances that have boring communities, but generally large alliances have good, active communities that keep lots of players in the game. Plus lots of large alliances offer academies and other helpful tools like economic aid etc that get players more interested in orbis and pnw in general.