Of course you will get fat if you eat anything in big quantities. But your post doesn't even make sense, you lost 50 lbs by eating Burger King. This obviously implies you are eating a lot less, not that Burger King is anywhere near healthy and better (or even the same) as properly made homemade food.
And yes, there are things inherently wrong with over-processed food like fastfood, high calorie density, they are nutrient poor, usually full of cheap carbohydrates (monosaharides) and trans fats. Which has nothing at all to do with quantity. You can still be skinny and unhealthy by eating only 1 burger per day.