I think it depends on the fish, too. I remember reading one person's message that some of her bettas did best in 2 gallons, when they always hid or fretted when in 5-10 gallon tanks. I saw that in two of my boys as well, they both had 5 gallon tanks, but Ed Gein was always hiding behind the heater. When I moved him to the 3 gal (swapping with Leroidesanimaux, aka Leroy), he perked right up and was a much more active boy. On the other hand, Leroy suddenly stopped making bubblenests and is much less perky. Of course, it could be the placement of the 5gal, which is in my sister's room, which rarely has people moving around in there like in my room.
Oddly, I just moved Ed Gein from the 3gal into the 10gal with the four ADFs (he'd stopped making bubblenests for quite a while now, and was now beginning to look rather dull), and right off the bat, there was a big bubblenest, and his sparkle is back. Heh, he confuses me. Sweeney, on the other hand, doesn't seem to care what tank he's in. He's been happy in the 3gal, in his 5gal (I have two), and in the 10gal (I think he was happiest in that one)