Why am I still skinny when I eat so unhealthy?

I have been looking for the answer to this question online, but I keep coming up with nothing useful. Most of it is about how "these people don't eat as much as they think they are". However, I eat junk food frequently and eat until I'm stuffed every meal, while also snacking every once in a while. My exercise habits are horrible, and I find myself doing no exercise at all most days. Does anyone have an idea why this happens, or how I could change my eating to put on a few more pounds?