Im sure most fish would be happier in the ocean than in a tank, and many reefers would love to be able to afford a 300 gallon system, but we all do what we can. I had to downsize to z 29 from a 90 and honestly there are things I like better about having a small tank, I am limited to how much I can put in it, but you can make a small tank look awesome.for a lot less cash, and it takes up much less space in a small home.
I think alot of things can be done outside some experts recomendations and work just fine. I know this fish may outgrow the tank in time, but for now he is very healthy, eating good, and has a nice home where no one picks on him, and butterflys arent big fast swimmers, he seems fine in the space.
Sometimes you get big fish. That just hide in on spot in a big tank I suppose they could hide just as well in a small one, as long as you keep them fed and the water well filtered.