Comment by paavoova

In your example, if you exercise, you can have the pizza as well as the sandwich. This can be significant, because suppose you have those 300 surplus calories 3x/week minus the exercise (not unreasonable, a small snack here and there, right?). 300 calories is about 3 bananas, so it might not even be unhealthy food.

Rough math: 3 * 300 * 4 = 3600 calories surplus/month. A pound of adipose tissue has ~3500 calories IIRC. So you're now gaining a pound of fat a month, and you're not even indulging yourself, really.

In reality, physical activity and diet aren't so steady, so some months you maintain weight, some you lose, and some you gain a lot. But over time it averages out, and you've put on 12lbs in a year.