How does limiting fats help fight against obesity and being overweight?

More calories/gram: Fats contain 9 calories in every gram while carbs and protein have only 4 calories per gram. So eating a food that is mostly fat will give you more than 2 1/2 times the calories of a food the same size that is mostly carbs or protein. For example, putting a tablespoon of butter or margarine on a slice of bread will give you 2 1/2 times the calories as a tablespoon of jam, which is mostly carbs.

Fats are double: Fats have double the calories per weight compared to carbs or proteins. A small handful of nuts or cheese can carry 300 calories. Hence, it is best to avoid. Also, the saturated fats are harmful in several ways.

Caloric density: Fats are high in calories, 9 calories per gram versus 4 for proteins and carbohydrates. So limiting fats in the diet help reduce total daily calories and helps one lose weight.