Are you referring to scale?
He has a point (though I'd never considered it before). The arena looks quite small really, and then we build temples and other structures....
But then, it has to be magic as it's impossible to fly out of it too
My interpretation is that the Arena is Colloseum Size - large enough for few big constructions. One Zone is big enough to fit a dozen Hydra-size creatures.
It's smaller on the outside...
The arena was created magically so that any magic used wouldn't affect the outside world. Why not make it so that individual zones can scale by themselves as needed?
Or these could be really small temples, ones that you can't actually go inside of.
In fact everything could be scaled down inside of the arena, so your foxes are actually ant sized, and Hydras are only dog sized.