Probably because of the number of calories you are consuming. Calories are the units used to measure the energy-producing value of food. They do not measure the nutritional value of food. Your body needs fuel (calories), but more important, your body needs the right types of fuel, just as a car engine requires the correct octane fuel or a boat engine has to have gasoline plus oil for it to run well. The body is a far more complicated machine than either a car or a boat, and essential to its efficient functioning are the kinds of fuel used to run it.
Calories come in three basic forms: protein, simple and complex carbohydrate, and fat. There are actually six classes of nutrients, although three have no calories. Each food has varying proportions of these six nutrients.
Energy (calories per gram)
Protein
4
Carbohydrate
4
Fat
9
Vitamins
0
Minerals
0
Water
0
We can see that fat is a very dense source of calories. Each teaspoonful of fat contains nearly two and one-half times the calories in a teaspoonful of protein or carbohydrate. A medium-sized baked potato, without butter or sour cream, has about 95 calories, all of them carbohydrates and protein. That same potato in the form of french fries contains 284 calories, and 119 of those calories are fat. Just 3.5 ounces of the potato chips Suzanne was snacking on contain 568 calories, 358 of them as fat!
It is easy to consume the same amount of food but, by changing the quality or fat content of the food, drastically increase your caloric intake and, in turn, gain weight.