How? How does it stunt it then based only on tank size besides physical damage? You have not addressed that issue.
Is it possible to keep a redtail in a 75g tank with pristine quality? How are you defining pristine water quality? Ammonia, nitrates, testing for other organics like metabolic byproducts? Let's say that RTC is 3ft long in a 75. It eats pounds of food every day. You can keep that water pristine? Well I know YOU can, but I mean other people.
Take that fish, put in a 75, plumb it to 1000 gallons with great filtration, that fish will literally grow to the physical boundary of the tank. It's very cruel and I sure hope no one would do it, but eventually that fish would be so large it would probably crack the glass and break the tank when startled.
I am the one that doesn't know what I am talking about? haha, you crack me up mate!
Anywho good luck with your puffer.