:shrug: It seems our current theory is: we're burning no energy while diving, so the only reason we breathe is to vent the carbon dioxide. It follows that a diver's gas mileage is inversely proportional to their respiratory volume, and once the diver stops sculling, being overweight, panicked, and all those other YOUR GONNA DIE!!! things, the size of their lungs becomes the only factor determining their SAC.
Problem solved, in two easy steps:
1. stop doing YOUR GONNA DIE!!! stuff,
2. get your lungs surgically removed.