[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.