It looks like you're describing human metabolism with the following model:
Cal(Food intake) = Cal(Storage) + Cal(Activity)
Is that what you're saying?
I'm pretty sure people excrete calories too. It is therefore not clear that simply reducing calorie intake with no change in activity will immediately result in fewer calories being stored in the body.
Yes, fat people nearly always eat too much but if you have a large meal after a period of fasting, for example, you're probably going to crap out most of the energy contained therein.