I thought they were fixing the spawns so they spawn all over the place?
If not can it be increased slightly.
When I last played the MA I tried using the wooden sword for the beginning few waves to conserve my bow which made people mad because 'I wasn't helpful'.

I honestly dont think the bow was the problem (got to 81 pre power 5 buff).
Its just that archers can sit in one of the corners and snipe away the more dangerous mobs, zombies and skeletons can be left to the dog.
I think the only way to 'fix' people getting so far is to put a 'minimum amount of players' kind of deal on a arena that large.

So for example make a smaller arena with no hiding places for 1-6ish players. Then for the bigger events use that arena, or an even a bigger one which I heard you were planning on making.

Or just add an outer strip along the edges of all the walls where every few waves mobs spawn back there, make it more interesting and active.
All just suggestions, ill still play the Mob Arena anyway.