I guess my point was that people stuff a lot of fish into too small of a tank because, well... they can. No one is stopping them. Just as there are many philosophies on how to set up a tank, there are many different philosophies on how to stock those tanks.
I have to agree with ziggy though - in my opinion, size of tank probably depends on the species of fish, and not its actual size at the current time. Just because a fish that's going to want a 100g tank is tiny for the first few months, doesn't mean you can put it in a 10g because it's only 10% of the size it's going to reach. I kind of think of it like raising a baby - just because it's small and doesn't eat much doesn't mean I'm going to put it's crib in a closet and only bring it out into a "normal" size room when it gets bigger. Yeah... probably a bad analogy, but so be it!
Oh... and great picture melosu! See - that picture PROVES the point that a fish will only grow as big as its environment allows! See... that shark obviously has stunted it's growth to fit in that "tank." I think it needs more flow though.