IMO, all this discussion on living space and comfort for fishes is based from a human perspective, and therefore, baseless from the perspective of a fish. as long as we can't communicate with fish, there'll be no fixed method of determining their happiness. after all, a glass box with clean water and a fixed temperature is a highly regulated and unjust representation of a natural habitat for a fish.
for one, the betta is genetically not suited to live in large volumes of water. i don't know how much truth exists in the paddy-field stories concerning a betta, but i believe any fish that has developed a labyrinth organ has done so because oxygen rich habitats are not a luxury to them. and having developed that organ, they have to come up to the surface from time to time to get oxygen. so the volume increase from 2g to 10g does little to further this cause.
also, personally, i don't know how comforting it is to live at a constant temperature all the year round. as long as the volume of water is large enough, the betta would be living at a temperature close to room temperature, which is quite comfortable.
and last, but not the least, the fish gets a constant food source, hard to come by in nature. this is the greatest luxury of them all!
again, the view varies from person to person as to what constitutes a healthy environment for their pet, and how much they are willing to sacrifice to provide those needs. but if someone falls short of one of these pre-determined parameters (namely space), they should be able the enjoy the hobby as much as anyone else with suitable types of fish.
