Taking junks once in a while will not make you fat outrighlty. However, making it a routine and eating them consistently will definitely take a toll on your weight. Like you said, they are snacks and adding them to your diet as snack time isn't so healthy. These so called junks are full of carbs, unhealthy fats and oils with little vitamins, minerals, and other elements. Some junks are processed food as well. When you have a diet full of these, you know you are getting less essential nutrients and more carbs that eventually turns into excess. Your body needs just about enough energy. So when you have too much carbs being converted into energy than the body can use, they are stored as fat hence leading to excessive weight gain.