Conversion efficiency is roughly as follows:
carbs --> 60% fat
protein --> 75% fat
fats --> 99% fat
So in order gain 1-lb of fat on the body, you can eat the following excess calories:
carbs = 5833 calories
protein = 4666 calories
fats = 3535 calories
You only need to eat a little bit of fat to pile it onto the body. You can get away with eating a lot more carbs & proteins before you pile on an equivalent amount of fat. Not to mention that fats are more dense to begin with. So an extra plate of fettucini alfredo will pile on 1-lb from fats in the cheese-sauce and about 0.25-lbs from the pasta.