If you have two arenas one which is 25000 and one is 20000. The first will sell 20 000 seats as fast as the second. But after the 20k if will be harder.
i ask forest in the "news" thread about it, and i also understand his answer that way ;)
Else it would be idiot, because the two game testing period we had before we start rebuilding our arenas would be with bigger arenas and some teams need nearly 2 season to get below those 20k^^