I think you make a very valid point, also possibly another reason why it's basically impossible for us to get Armatus much past 24", Goliaths past 36", and vittatus much past 24" as well. The same pretty much goes for every other characin as well now that I think about it. Tarpon (yes, I'm aware not a characin), Hoplias, Dorado, they all seem to not be capable of reaching their adult sizes in the home aquarium setting.
For the most part however I'm going to go out on a limb here and say that many here that recommend the larger tanks are far too used to seeing the bigger cats that seem to NOT be affected by their enclosure in terms of their growth. RTCs, TSNs, Channel cats, and Pangasius are just some that we all so commonly see stuffed in a much smaller aquarium only to see the fish continue to grow or end up stunted from the terrible water quality.
ETA: Also, I just want to make it very clear that many here that suggest larger tanks for these larger characins, and come down hard on those with smaller tanks do so simply as looking out for these awesome fish. Many that throw in their two cents have a vast amount of experience working with and keeping these guys for quite some time and know what these fish need ultimately to live as comfortable of a life as we can provide in a home aquarium.
Certainly you can go out and get a VATF, GATF, or Armatus and stuff it in a 180 and while yes, it probably could live in that space it all comes down to offering up something that might be comfortable for the fish. Simply due to the size none of these fish would truly be comfortable as at close to max size they would either be touching both sides of the tank in an attempt to turn or would have to bend a bit to execute the turn.
