The first alligator gar that I saw was not an especially impressive specimen. Nor was it a very big one. It was on display at the National Geographic Museum in Washington, DC, part of a special exhibit on giant freshwater fishes. The highlight of the exhibit was a series of life-size (meaning "enormous") statues of the various fishes. Any single one of them would have been an incredible fish to see in the flesh in the wild... or in a public aquarium.
As it was, the alligator gar was the only species on display, and, as one would expect for a temporary exhibit in a building not designed as an aquarium, was in a small, mobile tank, so had to be a small, mobile fish. Having a big one there would have been out of the question.
Working with fish and reptiles differs from working with mammals and birds in a lot of respects. One that has always fascinated me is the matter of scale. A lion is essentially a lion. A specimen at one zoo will probably look a lot like a lion at another. One my be a little bigger or smaller, but certainly they'll be close to one another. With fish and reptiles, however, the variation can be extraordinary. A Komodo dragon or a green anaconda or a saltwater crocodile may be several feet longer, considerably more massive. Bigger individuals can make for a far more spectacular exhibit. It's for that reason that the Bronx Zoo had an ongoing announcement, offering a reward in the tens of thousands of dollars for a live snake over thirty feet long. It was never claimed.
Another way in which reptiles and fish vary from birds and mammals is that, since they do not (usually) care for their offspring, juveniles - sometimes even at birth - can be displayed without their parents. This allows zoos with size constraints to display small individuals of species - an aquarium with limited tank space could display smaller sharks, for instance. In contrast, it would be considered extremely inappropriate and unethical for a zoo to decide that, since it doesn't have a big enough enclosure for polar bears or elephants to take a few young from their mothers and house them by themselves in a smaller enclosure.
The one issue with housing smaller members of a species is that there must be a plan in place for what to do with individuals as they grow into adults. Despite what many visitors believe, a smaller enclosure will not necessarily limit an animal's growth and keep it small. If that were true, we'd totally have elephants the size of ponies by now and SeaWorld would have specifically bred a pygmy orca.
I personally would prefer to make sure that any species that I house at my facility can be provided with care for its entire life, from infancy to the end of its life. I would never want to find myself holding an animal that I was unable to care for as it outgrew its environment and that I was unable to place in a more ideal situation.