It's hard to determine the correct dose on a huge body.  A large fit person doesn't have this problem. When you're a lard that needs to sleep with a CPAP, you're already experiencing pulmonary limitations. Sedative/hypnotic/anesthetic drugs almost always cause pulmonary depression.  It takes less drug concentration to kill them, so doctors generally undershoot.  In fact, this is why fats get rejected from non-emergency surgery and told to lose weight first.