That sounds simple, but it's not. The amount of calories you need drops with your activity level. So let's say an inactive person needs 1500 calories a day, and they meet exactly that plus a Snickers, which is an extra 271 calories. An active person who needs 2500 calories a day and does the same thing won't be hurt as much from that extra 271 calories; 18% of daily calorie needs versus 11%.

Being active gives you more "slop." Further, being inactive changes what your body does with those calories.