Fat is more calorically dense than sugar, and your examples are all low in fat as well - vegetables and fruits have more carbs than either protein or fats - and the high calorie examples (ie. Avacado) of those categories are high in fat. Fish has few fats and no sugars. The most calorific cuts of meat aren't that way because they are sugary, it's because they are fatty. It's just that fats lead to satiety quicker than carbs, but I'll bet if you ate pork belly for every meal you'd find that you could and would get fat on that.