100g of anything you eat is 300kcal to 400kcal. your basal metabolism usually requires 1600kcal to 2200kcal per day, based on your weight, just to exist. mcdonald's big tasty burger is 1000kcal(bigmac is 550kcal), about half of your daily calorie requirement and won't even make you feel full. in other words, it is REALLY hard to not consume more calories than you need and not get fat.