Its very important to remember when using basal metabolic rate calculators online that they are not accurate.  Fat requires almost no calories for the body to maintain and has a poor blood supply.  Muscle on the other hand is the opposite.  When you punch in "sex/height/weight" the calculator has no idea if you are 300lbs of fat or muscle.  The article said the 300 lb man had a BMR of 800 calories less than a normal man of his size.  The only way this massive deficit is possible is if they are comparing him to a bodybuilder.  Its very rare for a 300 lb man to have a normal body fat composition.