A whole egg is full of various nutrients while the white portion is full of source of protein. The rest of the nutrients are present in the Yolk. The egg yolk is rich in iron, Vitamin B2 B12 and D and cholesterol which are Missing from egg whites. If you eat only egg whites, you are missing out on other nutrients.
It is a nutrient dense food and not fattening at all as far as one cooks the egg in a healthy way