Your "calculations" are absurd, I'd say.
The Mind/Body Connection

My chess suffers because I was not born with the chess skill.
You've got the skill. But do you have the will?

Actually, phisically active individuals tend to perform better at activities, regardless of said activities' body or mind genre. The ongoing current of thought still enforces exercises will make you focus harder on chores of the mind as well.
Evidently, obesity and sedentarism are two separate elements, whereas an obese individual might indeed receive the benefits of exercising daily despite his overweight.
Overall, I suppose some exercise is bound to benefit us all and in several areas - not just chess. For the fun of it, imagine the following commentary:
PLAYER 1: Wow, that was tough...but I finally beat you!
PLAYER 2: Yeah, you did, but imagine if I lost those thirty extra pounds, pal! Know what, I'm starting my diet today!
PLAYER 1: Oh, noes!
It is no secret that the mind and the body are so closely connected that each affects the other. World Champions Bobby Fischer and Garry Kasparov knew the importance of being physically fit and both did a lot of cardio and carefully watched what they ate in order to be at their very best at the chessboard. There is an International Master who is morbidly obese with a FIDE rating around 2600. According to my calculations, if he ever got into shape, his rating would hover around 2850 and he would be World Champion! How much does your chess suffer because you are not in better physical condition?