I've seen those fish and taken the same fish and put them to larger containers too only see a drastic effect on the betta as a direct result of a larger tank.
For one, Small containers keep the finage wore right down, and does nothing for the bettas self esteem,
Try living in a small box you can barely turn around into let alone stretch out and run, Tell me that's not gonna effect your health, Just because some of us choose to look blindly to all the facts pertaining to these small containers does not mean it isn't shortening their lives let alone the unhappiness they must go thru every day of their lives. cramped in such small quarters,
Pretty much all health issues concerning betta's is due to small amounts of water and lack of water changes, Water changes in these small containers can only do so much & These fish suffer ALOT more sicknesses that oft times ends in deaths.
Take a look at the word "Breeders", that word alone can say so much, how about puppy mills, They house them in tight quarters to say the least... Point is their is no excuse for small containers yet we keep finding excuses or choose to be ignorant towards the facts of it....
I'm sorry.. I though we were talking about fish.. not dogs. How long have you been breeding bettas? Are you a member of the IBC? What experiences do you have with bettas that tell you keeping them in jars during grow out is detrimental? I know people who have been breeding SHOW quality fish that would highly disagree that jarring and cupping bettas as juveniles is bad for fin growth. It decreases the likelihood that they will blow out fins, plus the lack of flow from a filter allows for more natural growth.
I'm not trying to argue.. My point was merely to show that maybe if we stopped fighting for BIGGER and fought more for BETTER that eventually BIGGER&BETTER would start to come hand in hand. A fish who has quality food, clean water, and heat in a small tank is much better off than one thrown into a 5 gallon tank and forgotten about. Size is not the crucial factor :)
I only keep my fish in tanks 2.5+ gallons but when I begin to breed I will have fry in jars that will probably not be more than 1/2 gallon. Not many people have room to have 80 1 gallon tanks around.. not to mention doing the daily water changes for those.
Technically the container doesn't matter.. breeders keep growing fish in small jars all the time.. they just do the necessary number of water changes daily.
Don't forget that Bangkok, Thailand is the hottest city in the world and therefore water temperature is not an issue. Likewise, breeders customarily keep fish in heated fish rooms. That paired with 2 or 3 assistants performing water changes makes small containers possible for the short term.