No, it's not because "calories from greens don't have the firepower" - That's ridiculous. A calorie is a calorie. The reason why vegans have to "eat" more calories is because it's more difficult for the digestive system to break down fiber than fat, and so more the calories in the plant material are never absorbed by the body. That's why cow poop burns: Cows don't digest all the calories in their plant diet.