The bigger tank, the more margin for error you have. If you have a 10gal and your alk. level goes up 2 points in a 1 gal area, it could possibly kill some fish. If the same things happens in, lets say a 600gal. tank, well, I think the fish might notice something is up, but no harm should come to the fish.
I'm sure if everyone could afford it, we'd all opt for 1300gal systems. Why bother with the small nanos or even the larger 100+ systems when you can make the area look a whole lot nicer. Don't give him a hard time because he can afford what he likes. I can afford to drive a V8 Mustang (even with gas prices as rediculously high as they are) and I love owning one, so that's what I own. I won't settle for a Honda, Toyoda, Nissan, or VW because they have better gas milage.