I only have one betta right now and I'd love to have him in a 15gal, unfortunately mine sprung a leak (after spending ages scaping it
) so he's now in a 7gal... I won't be upgrading him in future, he seems completely and utterly content in his new home - I've never seen him so active. He used to be in a 5gal so the upgrade isn't too much of one but he just seems... happier. He's a beautiful, healthy boy and I love him to bits.
If I had multiple bettas I'd rather keep them in larger tanks (maybe 10gals a piece) and cut down numbers than more in smaller tanks but to be honest I only ever see me keeping one betta now after what's happened lately. I also find water changes eaiser on my single tank heh.
I'm sure there are many who keep multitudes of bettas in wonderful smaller homes and I'm sure that their fish are happy, healthy and will live long lives but I just want my one aquarium at the end of my bed that I can pour my heart and soul into... I'm too tired to take care of more than one anyway