Well in a crash, there's 3 collisions.  1- the vehicle 2- the contents of the vehicle, including you. 3- the contents of your body. An obese/overweight person will experience much greater forces in collisions 2 & 3 than normal and underweight people. The safety measures are only tested to around 200lbs, IIRC. So, it's insanity to think that going over the tested threshold by 100+ pounds will result in anything other than greater injuries or even a greater chance of death. That's just the limits of the human body. That's not even taking into account how the car itself would react to 250+ lbs impacting the interior. Changing saftey measure to accommodate persons of a greater mass could also result in greater injuries to average/smaller people as well.