Volume of the tank isn't as important as the dimensions of the tank.
Most ocean tangs travel more than 25 miles a day. They do this foraging for algae. Algae is low in protein so they need a lot of it to get the protein they need.
You put a fish that usually stays in a few cubic yards of water (like most anemonefishes) and defends its territory in a 30 gallon tank and it will do reasonably well. But put a tang that travels these great distances in a small tank and it will suffer chronic stress that will significantly shorten its life.
I see Yellow Tangs now in nano tanks. Sure the tang is small, and for that tang and the sailfin-type (
Zebrasoma) tangs that don't travel as great of distances, that can work out for a while. But for the 'traveling' tangs (
Naso,
Ctenochaetus,
Acanthurus and others), tank-life is stressful, but more so in a tank of short length.
It isn't they can't live there, it is just that their life is considerably shortened as the length of the tank is shortened. Take this guy who just lost his tang:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=842524
The other life-shortening factor is their diet. Most aquarists insist upon feeding them meaty products or brine shrimp. Neither is in their normal diet. The wrong foods will also shorten their lifespan. But with reasonable planning, diet and nutritional efforts like in this link:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=785228
they can get on well in a tank for many years, or 10+ years in a long/large tank. A Sohal tang I had once owned, now in another's care is going on 18 years so far and looking good. But it isn't in a 135 gallon tank!

Try 500.
Just my experience and opinions. It seems to be a matter of how long they 'should' be with us, and their quality of life while in our care. Almost everyone seems to have a variation on those issues.