Aside from "genetics" being the simple answer, larger frames can also carry more body fat and not look "fat". 18% body fat on someone who only weighs 140lbs would make them look completely out of shape. That is because they have very little muscle mass on their frames, assuming they are of average height. Someone who weighs in over 260lbs at 18% would simply look like they weren't that defined. They would still look huge and "built". This is one reason why your actual body fat percentage means a hell of a lot LESS than how much lean tissue is under it.
I don't eat pure crap when gaining, but I sure as hell don't avoid some food items that seem to send some of the people here into fits and convulsions. It simply isn't everyday or every meal. Along with all of that, the more overall body mass you carry, the more you can get away with in terms of food choices. If it takes 5-6,000cals a day for a larger guy to even gain weight, it isn't like that one hamburger with no mayo is going to set you back unless you are simply overly paranoid about gaining any body fat when gaining muscle.