I chose to build a Nano because I truly like the concept of working in a small scale and the challenge of a narrower water chemistry, but also because of affordability... In any case, even "IF" I was able to establish a higher water volume tank, I'd probably still keep my Nano.
If I didn't already have 3 nanos, I might upgrade to the 50-75 gallon range. Otherwise it's by choice. I do like small scale and it is easier to maintain cost wise.
For me,its by choice. I have had 75 gallon tank and down sized to a 55 gallon. I then went to a 29 gallon, and now I will be going down to a 13.5 gallon tank.
I have a 10g fresh, and a 29g jbj cube, mine is because of my land lord, I have concrete floors and still they won't allow anything larger than a 30g, and no more than two tanks, otherwise I'd have larger tanks.
By choice. I have a 90 gallon SPS dominant reef and then I have two nano tanks, a 20L that was originally a refugium that I built for my first tank, a 29 gallon mixed reef, and then a Oceanic Biocube 14. The 20L has been running for 11 years but has been made over many times. It's a really easy tank to maintain. I picked up the Biocube because I like the dimensions of the tank and it allows me the luxury of pursuing my love of the hobby in another room in the house.
I had a limit of 16 gallons in my apartment. I got the biocube to the max size. Now I am buying a house and will shortly be upgrading. BTW I had to take the nano down when I moved and had no space to put it back together. Two more weeks now before I can get the nano up again.