> the average American eats 400-700 excess daily calories

This can't possibly be true. A caloric surplus of 500cal/day adds a pound of weight per week. That'd mean in a decade of life the _average_ American would add an additional 260 pounds. In 4 decades Americans would add half a ton to their waistline, on average.

That'd mean at then end of their life the average American would die weighing over 2 tons

The question is how you calculate or define excess. For one, excess calories aren't 100% stored as fat. We're not that efficient.

Additionally the fatter you are the more calories you use at rest. So there's a point where if you consistently eat too much you'll stop gaining weight.

The biggest source of error here will be the calories in the garbage bin though. I wouldn't be surprised if a lot of this is stored in the waste disposal, not the waist disposal.

Assuming a steady surplus of 500, and considering that a kg of human fat is roughly 8000kcal, it will take two weeks to gain a kilo. But a larger person consumes more to simply stay alive so the curve flattens out once a certain mass is reached.