[QUOTE="This is an interesting idea but what counts as a small zoo and what counts as a big zoo?"/QUOTE]
[QUOTE="That said there are exceptions -Bristol has both of them and I'd still class it as a small zoo. Beyond that, and perhaps Wingham, though I'm struggling to think of anywhere that has great apes or pinnipeds which I'd class as a small zoo."/QUOTE]
Welsh Mountain Zoo, perhaps?
Anywho, I'd argue the simpler categorisation of 'small vs. large zoo' based on presence of ABC zoos isn't quite right - there's a much wider 'middle' category that fuzzies things up a bit. For example, though Bristol may be geographically rather small city zoo, thereby physically denying it the opportunity to exhibit more of the larger, charismatic megafauna (at least on their main site!), their financials and annual turnover, and, I think important to note, husbandry experience and degree of speciality puts them towards the 'middle-larger' end of the spectrum. I actually think that latter part is possibly a better metric to divvy up zoos in this sense (i.e. the more dedicated departments on separate taxa, the 'bigger' the zoo), regardless to some extent of what actual species are kept.
By and large, the smallest zoos have a single collective group of keepers charged with managing all their stock. Get a little bigger and you begin seeing a split between 'mammals', 'birds' and 'herps/inverts/small things in general'. Bigger still and the 'mammals' get split up into 'large' and 'small / primates' (à la Cotswold Wildlife Park), and the biggest zoos, at considerable cost for the additional staff expertise, speciality and upkeep costs, begin to have departments split into specific 'insect', 'fish', 'pachyderm' groups etc. ZSL and Chester are the obvious 'big' players here, but I'd argue Bristol is likewise more towards that end given their range of dedicated houses, and with livestock split across two sites.
In short, perhaps a better metric to define a 'bigger' zoo may be less about having rhinos, and more about whether they have a decent-sized aquarium or not. Or something!
EDIT: Hmmm, I can't seem to get the hang of the quoting formatting... !