It is common sense that if you intend on keeping a fish that is commonly known to reach a large size that you don't put it in a tank that is an improper size. By improper size I mean one unable to dilute the fish's waste in conjunction with your water changes and one unable to allow the fish some room to move about.
That is not common sense that is good ethical concern mixed in with appropriate fish husbandry.. I do not think anyone here is saying put a whale shark in a 20 gallon. What I am saying is there are conditional factors to take into account when housing any fish, you can not pigeon hole this into a blanket rule. What one may say is an improper tank size, someone else may be spawning their fish in. We really are not in disagreement here Arthur, I do know what you are saying, I do not disagree with you, but in saying it in a vague manner one can come up with 40 different exclusions that test it. Hence it becomes a dogmatic truism that is not exactly encompassing of all species needs or situations. Loads or Anecdotal and empirical information also clearly states that certain species do not reach sizes in captivity that they do in the wild. With a clear knowledge and understanding of the mechanics of fish growth and physiology (on a species basis) some of these hobby truisms really are kind of silly...(in some cases actually detrimental)
Well I guess in cases where you keep the animal improperly....
Again this is not true... Fish do not always live longer in captivity when compared to their wild average even when kept properly.. Some might, I'm not arguing this, but this is not a fact. Some species in captivity will burn out much faster than they do in the wild just because conditions are too good. Some good examples of this are with some Minnow species, Killifish and Livebearer species and of coarse my favorite the Lamprey

.. Again you can not blanketly say all species will react to fit the same pigeon hole we give them...