Plus, muscle needs more calories than fat.  One pound of muscle burns almost 10 calories a day, whereas one pound of fat burns 2 or 3. Source. So if you gain 30 pounds of fat, your bmr goes up by 90 at most.  The same amount if you gained 'just' 9 pounds of muscle.  Obviously, the latter is going to be much tougher than the former, but still, all the more reason to get at least a little swole.