I don't think he meant that it is linear (i.e. the larger the wheels, the more...) but rather there is an optimum size of wheels (which translates to distance) per the amount of power being applied to get the most mpg. Evidently, his Sprinter had a low rear end (many revolutions of the engine to make one turn of the wheel), and the Transit with a high rear end (relatively less revolutions to make one turn of the wheel). Think of it more like a bicycle: If you put larger wheels on it, then you will need to shift down into a lower gear to get started rolling. The pay off of larger wheels (or a high rear end like the Transit) is that once you are up to speed, as long as you aren't too heavy you can cruise really nicely without much work, but if you put exaggerated huge wheels on it, you end up hardly able to pedal it without massive amount of huffing and puffing.
I hope this helps. Maybe someone can explain it better or let me know if I'm off.