I wouldn’t be surprised if most thin people who claim to eat lots of junk food actually just eat a lot less regular food. In other words, they don’t eat many calories, but the calories they do eat are junk.
I assume you mean a bar mostly consisting of dairy and sugar with a bit of chocolate -- making a meal out of actual high quality dark chocolate adds up, but can hardly be considered junk food. <3 my 90% dark Lindt chocolate bars -- between those and all the bacon I've been eating this year the pounds are falling off.
I would say this was partially true in my case. While I ate badly, I didn't eat huge portions regularly. I also didn't eat breakfast regularly then (and often don't now either). In college we got fish and chips takeaway once it twice a week,and while it was definitely the worst kind of junk, I would be full quicker than others. That said, I regularly put away a quarter pointer, portion of chips, fizzy drink and a cheese and onion pie (deep fried cheese basically, and amazing). And not an ounce of fat on my body.