Oh the mini zoo is a good idea. Would have been even cuter to have an official in game text pop out " Cute Monster Zoo!"
I haven't tinkered with this yet, but theoretically couldn't you increase the chance that a zoo spawned first and took up more real estate?
The only chance to make large zoos spawn more commonly is to create many variations of them (so that there would be a higher chance of the game trying to create one, and that they'll have a higher chance of fitting due to there being many versions), but that also increases the chance of getting multiple zoos per level, which makes it a less-than-perfect solution.
...unless there is hard coding that only one Monster Zoo can spawn per level which I think may have been mentioned.
*COUGH* ...Man. That was a really bad cough! I should take a cough drop... or drink some water... or something...
