Processing increases the calorie availability of food which means a greater percentage of calories end up in our body. Beneficial for weight gain, terrible for weight loss.
The act of food processing is like a form of pre-digestion breaking down complex foods into easily-absorbed energy. Sugar, refined carbs and oils (the staple ingredients in most packaged foods) are the primary culprits. This is one of several reasons why weight gain is so much easier and more common on a highly processed diet.
On the flip side the calories in whole-foods are less readily available and require more work from our digestive system to access. Coupled with high nutrient density and its a weight-loss home run!