i dunno.... i think its because when the arenas are empty, having just one guy on one side and lets say 10 on the other means that one guy can earn 100 perks per sortie etc.
Whels i think earned over 1000 perks (it was 1000 wasnt it?) in one sortie because of this.
however changing this is odd.... the change should be to either close empty arenas once offpeak or to try and get players into these arenas..... changing the ENY bonus isnt going to fix anything.