Some people who have problem putting on weight don't put on weight by eating more if it's empty food that doesn't contain nutrients even if it contains carbs, fat and protein. Burgers contain all the above. Normal people would still put on weight but some skinny types just burn off all the calories unless they eat whole foods and special combinations of foods. And some have high blood pressure or ciruclation problems at a young age from fat in their arteries because they eat too much of it trying to put on weight. Excess fat from the diet can be difficult to store in adipose tissue and alot of it can end up stored in and around organs. I have met one or two people who had heart problems in their twenties and found out by chance when going for a regular checked up.
If Letsgoflyers is one of those types he should do some research on special diets for ectomorphs/underweights.
I guarantee that if he ate 4 1000 calorie burgers + fries and soda every day he would gain weight... and that is all empty calories.