There's nothing even wrong with eating fast food. It's just that fatties eat a bunch of it.  On the weekend I got 6 spicy wings and a serving of fries from KFC. Knowing I would be drinking a bunch of booze, having some food on my stomach was important, but the keyword here is some. When I felt full I stopped eating, so I had 2 wings left over for breakfast. A fatty would think "teehee; it's only two" and eat them, then buy another meal in the morning.