Note that as mentioned in the link, muscle only burns 6 calories per pound per day, which is more than fat but much less than popularly believed; some people say muscle burns as much as 100 calories per pound per day, which is impossible when you consider than your body is about 40% (skeletal) muscle. Of course, you can exercise that muscle and it will burn more calories.
Which does not quite answer the question directly. What it tells us that most of the variation is based on FFM (which is more than muscle) and that additional fat mass varies BMR 1/5 as much as FFM does, on average. But that lots of BMR variation exists for no as of yet clearly understood reason.
Here is another stab at it, specifically this table. That approach comes up with fat mass contributing 4.5 KCal/kg/d which confirms the 2 KCal/pound number and also may be the source for the 6 calories per pound of skeletal muscle. Kidneys, heart, and brain seem to be the big calorie burners on a per pound basis.
