Eating habits vary considerable among people. It appears that our eating patterns are designed to defend against eating too little more vigorously than eating too much. The researchers concluded that the lower the correlation coefficient between the energy content and the mass of food intake, the higher the BMI.
Energy content is the number of calories, so the only way to achieve a low correlation between calories and food mass is to eat lots of high calorie foods with little mass. Sugar is the epitome of this.