Inches of fish/gallon rule -- USELESS!

I have frequently run across guidelines for the number of fish that
can thrive in a given size aquarium expressed in inches of fish per
gallon. I trust the following assumptions are reasonable:

*Oxygen consumed and waste produced are roughly equivalent to body
mass

*Oxygen consumption and waste production are the primary factors in
determining minimum healthy space requirements for fish.

*The proportions of a fish are similar regardless if the fish is one
inch long or ten inches long.

OK. Body mass is equivalent to the 3rd power of the fish's length.
So 10 fish 1 inch long have only 1/100th the total body mass of 1 fish
10 inches long. Yet according to the frequently quoted rule of thumb
they have the same space requirements! Let's use a little less
extreme example: 10 fish 1 inch long = 10 arbitrary units of mass. 5
fish 2 inches long = 40 of the same units! So we a have discrepancy
of 4/1! That's still way more than enough to render the rule useless.

Is there something I am missing? Seems to me we need to add up the
length of each individual fish taken to the 3rd power and only then do
we have a usable guideline.

Don