This is why I was thinking of giving the PK from you Pixel, a 5g tank to himself. I am sure he'll love all that room to swim since he won't have such a long heavy tail to carry around lol But my long tailed boys will stay with the smaller tanks. Actually my only HM, Spidey, doesn't use much of his 3.3g but he was so active in his 1.5g.
I think Bettas build bubble nests out of instinct. They build them when conditions and water is favorable for breeding, not because they are happy. I truly don't believe any Betta would spawn and raise fry in conditions that are not suitable. Like most creatures on this planet, they breed when offspring will survive. Bettas will make bubble nests in a 0.5g tank yes, but that's usually after a water change. Don't keep up with the water, no nest. I usually change my tanks when my boys stop building nests LOL This all what I think though, not scientifically proven at all.
Mr. Plum is a HMPK, that's why I got him the 5g. He is very active in his small tank but, when i put him in the big tank he stops swimming around. Not lethargic, just not as active. He's a weird little fishy...
my males prefer larger tanks. my deltas were never happy in smaller tanks at all, and loved the space of their 2 gallons. i may experiment and use Ichi in my 10 for a while, just to see how he likes it. if he likes it, i'll upgrade him. if not, back to his 2 he goes.
personally, i hate longer-finned males in smaller tanks. they can see their tail easier, which will cause them to tail bite.
i do, however, have a Plakat female who hates anything larger than her one gallon. she's an odd case, because she's so cramped in her one gallon, but she's happy in it. i've tried her out in a 2 gallon, and a 1.5 gallon, and she hated both. strange gal...
I think it really just depends on the fish. I have some that have special needs and need shallower water than most. My King is happy in his big tank, but my male VTs esp seem happier in 1-3 gallons. My females are more active than the males. But I don't have a sorority. A lot of my females will rip each other apart.
I also think that the shape of the tank has a lot to do with it. I have an AA Panoramic (5 gallons), which is very wide but not massively deep or tall. It gives my halfmoon Odysseus a lot of lateral swimming space without him needing to fight too hard to get to the top, plus the filter is really gentle. He's absolutely bloomed since I put him in there. :) Now I just can't imagine him in a smaller tank (though since coming to this site, I no longer think 1 gal tanks are of the devil and have no problems with people using them as long as they are clean and warm. :) Thanks for opening my eyes, bettafish!)