It would be great if more Americans knew this stuff. In general, it usually comes down to the sauces and dressings. For example, a cheeseburger with lettuce, tomato, pickles and ketchup is generally going to have fewer calories than a tuna salad sandwich made with mayo. "Dressings" like ketchup, mustard and soy sauce have no fat and very few calories. Mayo, cream cheese, high fat salad dressings and the like are almost pure fat, and therefore have tons of calories.
(And always check the fat on turkey burgers! The turkey they use to make the burgers is generally not the same stuff you put on your sandwiches. Sometimes you're better off with a lean beef burger.)