Hi All, I’ll need to address this post in stages as it’s a prime example of how people can use confirmation bias and inferences to validate an ideology.
[ I'll respond in stages as well. 
First off, I posed the question as a conversation starter not as a promotion of one method or another. But you have to agree, fish growth is different in the two systems.]
Firstly, here we have assumption. We are using size as a metric for determining fish health. Who decided that was the metric in the first place? First we must determine if this is the correct metric for fish health before we start measuring it! This is like using lumens or watts as a metric for determining plant health and growth then saying these 120W tubes are far superior to this 80W for plants growth when the true metric is actually PAR. Suppose we argue that size is the best metric for determining fish health. How do we know what exactly it is about water changes that helps them to achieve their ‘proper’ size? How do we know if it’s the removal of hormones or nitrates that are causing an issue? Perhaps it’s the addition of something that helps? The answer is we don’t know of course. I don’t think people are actually listening to themselves when they imply that hormones are stunting fish? I mean, there truly is no evidence to support this notion whatsoever so really, why do people say it? It shows how some people believe everything they are told. But the reason they believe it is because it supports their philosophy. Even if they have never seen any evidence to support the claims. The will stick to it and try to falsify everything else.
[ I refer you to this in reference to hormones and growth: https://thefishvet.com/2012/02/28/do-goldfish-grow-to-the-size-of-their-tank/
and this with the extra documents at the bottom: https://pubmed.ncbi.nlm.nih.gov/11048680/ ]
The next question here is what is ‘undersized’ and are we only comparing this with other breeders? If so then we need to look at what other factors were involved with the no water change method as opposed to the water change method. For example, how much does the water change breeder feed? Do they need to perform regular water changes because they feed so much and does the amount they feed have any bearing on their size. What were they both feeding and how often? In no water change systems feeding may be less to avoid a deterioration in water quality? What are the sources waters of both locations and how stocked are the fish in both tanks? Is it generally true to say most species will grow larger in captivity? And if so how much can this be attributed to having a regular food source? How do the sizes compare to their wild counterparts? And are tank bred fish healthier than their wild counterparts? Again, what is the metric? [
Growth has been the standard of measurement for most if not all species, including humans. The determinations of what the species "should be" is based, I believe, on an overall average of a large population of the members of that specie. So when a herd of mammals is studied or a school of fish is studied, more than 50% must be of similar size at the same age to gain that average. These are also, I'll admit I'm assuming this, based on wild animal populations negated issues like overcrowding of a tank or excess or reduced feeding issues. This is an interesting piece on the effects on growth when food is reduced or eliminated for periods of time: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3696842/ ]
Now here, without a single one of those questions answered or even considered we immediately go after nitrate. We also see that quote again regarding ‘maxium size’. Again, how do we know that size is equal to health and what is maximum size? Do all Discus grow to the same size? Chickens are made to a saleable size very quickly in captivity, this is certainly not natural. Are they healthy? But chickens are not fish right?
[ I brought up nitrate because of it's constant presence in the aquarium ( vs temporary ammonia and nitrite) and it's known toxicity to life. The point I posed was more along the lines of If everything else was equal, would a fish in the presence of higher nitrate levels be of equal health to one kept without the presence of nitrate? If growth is the standard, it doesn't appear to be. I also didn't say it was the only reason for the lack of growth. Regarding chickens, chickens that are raised quickly and have been made so over plump that they can't walk or fly would be considered unhealthy ( compared to the majority of the chickens in the same coop) yet are deemed safe for people to eat. This might interest your chicken intuition. "The lives of broiler chickens are not much easier to contemplate than those of the egg-layers. Much research has been devoted to genetic selection to produce the most economically efficient bird. The RSPCA produced a pamphlet several years ago that for me still provides the best illustration of what this means for the chickens. A series of photographs taken a few days apart showed a normal, traditionally bred egg-laying hen as it grows from chick to maturity. Underneath were parallel pictures of the modern broiler taken at the same intervals. By day nine, the broiler’s legs can barely keep its oversized breast off the ground. By day 11, it is puffed up to double the size of its cousin. It looks like an obese nine-year-old standing on the legs of a five-year-old. By day 35 it looks more like a weightlifter on steroids and dwarfs the egg-laying hen." The complete article can be read here: https://www.theguardian.com/environment/2016/apr/24/real-cost-of-roast-chicken-animal-welfare-farms
Here again we have assumptions the fishes life MAY also be shortened. May it also be lengthened? Where is the evidence here? This is just a general comment to support ideology and convince people that what you’re saying is actually true.
[ No, it was more a point I posed to the group to see what the experiences here have been. If a 1 year old neon Tetra died at only 3/4", I'd say, no matter how healthy it looked or acted, that it's life was shortened compared to the 3-5 year average for the specie and the 1 1/4" - 1 1/2" average maximum size which it usually attains within the first year from egg. ]
You cannot compare even a public aquarium to home aquarium for a whole host of reasons. Again, size is referred to as the metric with no actual evidence to suggest that this means the fish are any healthier.
[ I have to ask, Why not? Are their fish being kept in a tank? Yes. Are they feeding their fish? Yes Do they filter their water? Yes. What do they do differently than the average hobbyist to get the results they are getting? Public zoos and aquariums have a more strict guideline to follow for their animals so getting their results should the goal of every hobbyist as it's usually the best care.]
And again here. Bigger is always better right? I suppose it makes sense so it must be true.
[See previous response regarding size.]
Well I don’t know, you are trying convincing me that there is a problem to begin with and now all of a sudden my fish aren’t growing at all! Gulp! And let’s throw in some anecdotal association for good measure because it’s my philosophy after all.
[Not at all. ]
And now for the piece de resistance. ‘You might have a healthy looking tank with growing plants with acceptable water chemistry and your fish might look happy but they still suck because mine are bigger than yours and I do water changes. Finger point. Are you really doing your fish a good service???
I mean, it’s a great pitch. But this is basically everything that is wrong with this hobby. People always saying things without actually asking the real questions. Assumptions and inferences. It’s what this hobby has been built on since the 70s. The science in our field is lacking so badly it really is no surprise. I wonder what the source for hormones stunting fish growth really was. When it first started. Man that one has evolved and grown three heads. Show me a study or an ounce of evidence and perhaps I’ll change my opinion. But you can’t. Probably never will and despite this being the only fact in all of this I know that this will fall on deaf ears and run straight off the back because people have ideologies, methodologies and philosophies and there can only be one way right. WRONG. Forever wrong.
How very kind of you. There are some outstanding tribal elders here.
You’re the best, Colin T!
You got that one right, Joe D!
) There is a lot of misinformation out there and a lot of it is in online fish groups as well as the people we now depend upon in stores to know what they are doing. Just look at the hobbyist that is keeping his 10" Oscar in a 30 gallon tank for 5 years, with hole in the head clearly showing, an undersized filter and he's giving advice on how to keep fish. They are out there. 
