What cases can you think of where is city is relatively large but their zoo is relatively small? Paris - Zoo is only 15 hectares and 180 species. Osaka - 11 hectares and 200 species Mexico City - 17 hectares and 250 species (small for a city of it's size) and many more... Note: I did not include London because it also owns the exurban Whipsnade Zoo or Tokyo because it has the suburban Tama Zoo.
Indianapolis has a surprisingly small zoo. Tokyo may not have a large zoo, but Tokyo has many, many, many smaller zoos.
It also has the Tama Zoo, with 50 hectares and 350 species, which is quite large. Plus the central Ueno Zoo isn't that small either.
Of the 25 largest metro areas in North America... -Montreal doesn’t have a “standard” zoo at all, instead having two small special-focus institutions: the Biodome and the Ecomuseum Zoo. -The Riverside/San Bernadino area technically has the Living Desert Zoo but that institution is >1 hour from the center of population. For the remainder of the top 25, two have what I’d consider fairly small zoos as their main zoo: -Atlanta. Zoo Atlanta is on a compact site, has a good herp collection but otherwise smallish collection. -Boston. Franklin Park Zoo is not tiny in area but there is lots of unused space and the collection is small). Philadelphia is also on a small site like Atlanta, but the collection is broader. Other big cities in North America that don’t have a major public zoo (i.e., excluding special-focus zoos/aquariums or private zoos) are Vancouver, Charlotte, Las Vegas, and Austin.
Austin, Texas is a major city that doesn't even have a notable zoo. I know of a few that are like sanctuary-zoo hybrids, where they operate like zoos but focus primarily on rescue animals. Maybe I'm mistaken, but I feel like a traditional zoo could do well in Austin. Doesn't have to be right in the middle of the city or anything.
I do not know if what birdsandbats said about many small zoos is accurate but the Inokashira Park Zoo is a pretty small zoo. The city of Yokohama also has three small zoos. I don't know how big they are but they don't really have a large number of species.
Paris has 2 zoos and 2 aquaria within the city and a safari park, a generic zoo, and several specialist collections within 40 km of the city, so while the main zoo in the city might be relatively small, combined there is a lot to see.
Lyon, France. The second biggest city in France and has a really tiny zoo. Just few hectares, 62 species and 5 or so exhibits, although modern ones. Marseille, Geneva, Bremen, Szczecin are sizeable cities without zoos to my knowledge.
Brussels, while it doesn't have a zoo in the city itself, is pretty close to Planckendael (which has signs in both French and Dutch).
It really is surprising. Many small and mid-sized cities have much larger zoos. I know space is often an issue in the big cities, which is why many build "second zoos" for the larger animals to roam (EG London has Whipsnade, Tokyo has Tama, Chicago has Brookfield, Antwerp has Planckendael, etc.) Maybe Mexico City should do what other large cities did.
Also. Lille is not particularly large, but a zoo on 3.5 hectares containing 100 species is quite small for a city of it's size (1.5 million in the Lille area).