Eating a vast amount of junk despite overeating...

Can this cause 'underweightness',
Sorry, I know this forum is primarily catered to those who're looking to lose weight, however, the board is 'general diet' aswell.
Basically, if you're eating 70% of your calorie intake as junk such as chocolate bars, sugary cereal, basically empty calories, woul this cause a low weight? I always thought that calories were calories and since I'm eating such high calorie foods, I'd have thought that if anything, I'd be in the healthy/ perhaps overweight catagory for my height.

Any thoughts???