Do you ever wonder why people who are ripped can afford to eat more than you?
"Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest." Your BMR assesses the amount of energy that a person needs to keep the body functioning. Breathing, circulation, temperature regulation, cell growth, brain and nerve function, and muscle contraction are all included. BMR affects the rate that a person burns calories so indirectly also affects whether that individual maintains, gains, or loses weight.
Increasing lean body mass can increase your BMR. The human body naturally burns more calories the more muscle mass you have. One pound of muscles burns approximately 5-6 calories per day at rest. It's not much, but if you compare that too fat, which burns about 2, you have a 300% increase. Not only does the actual muscle burn calories, but your body is forced to burn more calories as well just to maintain that muscle.
So, when you notice that serious weightlifters and bodybuilders often eat massive amounts of calories, it is because they need to in order to get enough energy to keep to their routine, but also because they can afford it because their bodies can naturally burn away excess calories. Gaining muscle while losing fat will improve your body composition, your BMR, and your attitude.