They can get bigger and do have the potential to do so. This should not be ignored or discounted, it is just not accurate to expect it to occur, These are not average sized fish often being quoted for size. They are record sized ones and not representative of the population as a whole.
In some populations some fish do get a lot bigger than in others in disjunct populations. Some fish kept under certain captive conditions will grow faster and larger, some will grow average even under these conditions.
I'm personally sick of the "this fish needs this size tank" stuff... This is not accurate in practical husbandry. What you need is a tank matched to the size of fish you have and with gar this is critical. You cannot have too large or too small of a tank for the given fish. There is no clear cut rule. It is all individual. To keep these optimally it is desirable to have multiple housing options available.