In my opinion, it all depends on the fish. Merlin was actually very happy in his one gallon, but that was also because he'd never known anything larger. Now that he's in a split 5.5 gallon, he wouldn't go back to a one gallon. Dragoon on the other hand, became a bit more active in a one gallon while he was in quarantine, but as soon as he got a larger space he really perked up. As several people have said it really depends on the betta and how much effort the owner is willing to put in. I couldn't handle the upkeep of a one gallon, which is why I have the 5.5 gallon.
"Animals are reliable, many full of love, true in their affections, predictable in their actions, grateful and loyal." ~Alfred A. Montapert
My Bettas: Dude-Male Salamander HMPK & Molly-Female Marble HM
My Goldfish: Rory
S.I.P. Beta-Blue & Black CT//Dragoon-Dark Copper DSPK//Merlin-Red VT