I have spoken to Scott on several occasions about the minimum tank sizes in his books, and he has told me that many of them, especially for the larger fish, are smaller than what he would keep those fish in. One reason is to not undermine his own information. If he says you can't keep a yellow tang in a 75g tank, it won't be long before someone comes along and says, "See, I've keep this tank in a 75g tank for years, proving that there's no reason to trust anything in Scott Michael's books." Of course, leaving aside the ridiculously fallacious reasoning there, it's just going to create more hassles than it's worth, and it would serve to undermine all of the other good information in his books.
I have a hunch there was also some pressure from stores, etc to not make the tank sizes too big, because then they'll have a harder time selling fish, but I don't know that for sure, and I think (and hope) that Scott wouldn't bow to that kind of pressure, though it's possible the publisher would.