There are on average between 55 and 80 calories in an egg depending on its size. The calories in an egg will vary depending on the size and how it is prepared.However, on average a medium-sized egg will contain around 70 calories, an average small egg will contain around 55 calories and an average large egg around 80 calories.Most of these calories come from the yolk, which is the most nutritious part of the egg and includes essential vitamins and minerals.he way you prepare your eggs will of course affect their nutritional value. For instance, if you fry an egg in fat, instead of boiling it, this will increase the number of overall calories. It is also important to eat your eggs along with other nutrient-rich foods, such as vegetables, salads and whole grains.