Right. When you weigh a certain weight, you have a certain resting energy expenditure. Let's say the average 200lb(Sorry for the American units) man burns 200

Right. When you weigh a certain weight, you have a certain resting energy expenditure. Let's say the average 200lb(Sorry for the American units) man burns 2000 calories a day. If he then goes a week only eating 1500 calories, he will have burned a pound, and now only weighs 199lb, and his new resting energy expenditure will be slightly less. The next week when he is still eating 1500 calories, he will lose only 0.99 pounds. So of course you will run into a time eventually when you plateau. At some point 1500 calories a day will be your new equilibrium weight and you will no longer lose weight without additional changes. This is typically an argument the other way, as when you do the math this way it's very clear that the energy when stored as fat is incredibly energy efficient. An extra 100 calories might mean that you gain a pound in slightly more than a month, but you won't settle on your new equilibrium until you gain more than 10 pounds. According to wolfram alpha, the basal metabolic rate of a 200lb male is 1997 calories. The basal metabolic rate of a 300lb male is 2465 calories. The daily energy expenditure of a 200 pound male, who runs daily for 3 miles over 30 minutes, is equivalent to a 300 pound male. 100 pounds of fat is actually a shockingly small amount of daily calories difference. Anecdotal evidence: I've lost 70 pounds, then regained the 60 pounds, then lost the 60 pounds, and am currently up 30 pounds. Simple does not mean easy.