The belief is once you get over 20k you can only fill half at best of the next 1k over that. The max is 20,500 who can ever turn up to a game.
he has 24500 seats and went 50 over the 20500. hardly a return on the investment. I sure if you had a 40k arena and set all your prices to lowest in top level league in the finals you would get decent amount past it but in real terms anything over 21k is a complete waste of money