Recent studies from Cornell University show that the calories consumed in "low-fat" foods lead many people to eat 28 to 45 percent more calories than they would if they were eating the full-fat version.
There's a very simple mechanism at work here. The human tongue predominantly tastes only fat and sugar. Protein is rather bland without seasonings and the most delicious cuts of steak are also the fattiest. The more sugar in a fruit or vegetable, the less bland it is. When you make something "low fat", the fat has to be replaced with sugar or no one will eat it. So "low fat" is often higher sugar than the original product, but that's okay with some people because "it's the fat that makes you fat." Of course that's a myth, but nevertheless. The reason people eat more is because sugar stimulates the appetite. Once you stimulate those taste buds, it is very hard to turn down eating more. I have the same problem; I can willpower my way past the cookies and fudge and candy for days. But if I have "just a bite," one bite becomes ten.
It's an evolutionary mechanism. We evolved in an environment full of protein (animals), but relatively sparse in terms of fat and sugar. Wild animals are only fat at certain times of year and plants only flower and fruit during certain seasons. Those that could find the high-energy fat and sugar were most likely to pass on their genes and survive.
Naturally, the healthiest thing to do is to avoid snack foods. Special occasions, sure. But don't make Wednesday night a special occasion just cause it's a Wednesday. And when you do eat snacks, eat the original version, which is the one you really want anyway. Eat a bit less of it and call it a day. The fat will help to satisfy you anyway.