15 kg of muscle (not just water and the accompanying fat/glycogen/bone density that comes from being that strong) is a lot. That might represent a 30 kg difference in actual body weight. For someone that big, an extra 250 calories per day (fat is metabolically active, too) is pretty insignificant compared to the immense calorie burn they’ll get from the actual exercise needed to maintain that muscle.

Like you mention, a huge chunk of the effect is the increase in calories of just moving that heavier body around in daily activities.