If you sit on your ass and pound soda all day, as long as you're not consuming it alongside a fat source, it isn't going to be efficiently fattening. When you consume it alongside a greasy cheeseburger and fries, NOW you have a recipe for massive fat gain because your adipose tissue will suck up all that dietary fat and you won't burn any because you'll be burning the sugars from the soda preferrentially.
Eating a diet high in protein and carbohydrate but with no fat isn't an efficiently fattening diet until you throw dietary fat in the mix. It happens to be that fattening foods are high in fat AND carbs - french fries, brownies, cake, cookies, doughnuts and chicken alfredo pasta all contain as much fat as they do carbs, so you burn the carbs and store the fat. It would be very difficult to get fat eating a diet of soda and pixie stix in the absence of all dietary fat. The average male has to consume something like 700g of carbohydrate a day before de novo lipogenesis (conversion of carbs to fat) is significant in the absence of dietary fat.
But how likely is it you'll consume 20 cans of Coke a day and zero fat?