The chubby little padded part gets even chubbier when you gain weight. Therefor, more cushion for teh pushin as they say hence, lesser penetration.
When you lose that chubby part women can get more direct fun rubby action and when dudes slim down their chubby part they can maneuver better too. That must be where the Doc gets his calculations.
I'm no mathematician so I don't know about the whole "1 inch" promise but can attest to the fact that things are much improved when one gets rid of that chubby for the rubby.
wow.