[oezi]

I wouldn't say 400 extra calories is modest, if you consider Calories in/Calories Out then even a modest increase in calories is leading to a continued accumulation.

By the authors examples eating 1.000 calories extra for 10 days gives you 1kg of weight gain. Assuming that the average diet went from 2.000 to 2.400 calories, it makes sense that people should gain 400*365/10.000 = 15kg per year until that extra body mass is increasing base level energy need or people are more physically active (but 1 hour of physical exercise is only 600 calories for many sports).