Muscle mass, itself, plays a big part in metabolism. Two people with similar builds, weighing the same, but with very different body compositions (say a weight lifter versus couch potato) will burn different amounts of calories both at rest and when performing various physical activities. The weight lifter going on a 5-mile walk will, typically, be burning more calories than the couch potato. Even just overnight while sleeping, the weight lifter will be burning more calories.