I'm the same way. I eat a crap load of stuff from McDonalds, eat a whole bag of Cheez-its, drink five to six cans of pop a day, and eat other greasy food and I'm not overweight. I hardly exercise.
Though, I know it is going to bite me in the ass someday.
Well, it doesn't matter how much pop you drink. It doesn't have fat in it, lol. And if you eat the same thing everyday for a period of time, such as fast food from a particular place, you won't necessarily gain weight. If you eat a lot, your cholesterol will go up, but your body becomes so used to the same food, it digests it easier and you won't gain weight. Almost like it becomes immune to the fat in whatever it is you're eating.