Quite often you'll hear the adage that a pound of bodyweight equals 3,500 kcalories of food. Well, this would be true if you were a bluefin tuna. New flash: you're not a fish. Your metabolism does not function like that of a fish.
Certain animals like the tuna grow until they die. Their muscles grow bigger, their bones grow bigger. Here we have a chart that shows the growth pattern of the southern bluefin tuna:
This looks nothing like the growth chart of humans. Note that the age of sexual maturity for the bluefin is about 4 years of age. Note that tuna start out life shorter and lighter than humans but they end up bigger and longer. In fact, in some parts of the world the largest bluefin tuna get up to 9 feet long and weigh near 1,000 pounds. They are eating and growing machines, converting sardines and anchovies and squid into ever larger tuna. The only limiting factor seems to be the availability of food; if you feed a tuna more food, you basically get a nearly perfectly upscaled version of the same fish. Their allometry at any age is described by this very reliable formula, well known to avid fishermen:
Length x Girth² ÷ 800 = Weight
Obviously, formulas to predict weight based on girth and height simply don't work on people. Humans and most other mammals have quite a different metabolism. Humans stop getting taller soon after sexual maturity. Then they live another 50 or 60 years at the same height and -- until recently -- more or less the same weight. Look at other mammals in the wild -- they too stop getting bigger sometime shortly after sexually maturity. It's not just height that stops; weight gain usually slows down dramatically or stops entirely. Mammalian metabolism has a built-in weight regulation system that is designed to keep adult weight within a certain narrow range. The mythical 3,500 calorie formula simply doesn't account for this. This is in stark contrast to animals like tunas and pythons and crocodiles, who grow until death.
It is only when you start feeding mammals a "modern" diet filled with sugar and processed food that
weight gain continues ad nauseum. Importantly, even this type of adult weight gain is entirely different in character to the tuna. If you feed a bluefin tuna more food, the result is more muscle and bone. If you feed a human a bunch of crappy modern food the weight gain will be mostly fat, some muscle, and almost no bone growth. It is only when the bodyweight regulation system is disrupted by crappy food that weight creeps up. Tuna have no such bodyweight regulation system -- for them more calories equals more fish. Tuna never stop being hungry. For you, more calories now should mean less hunger later. It would too, if you weren't eating such crap.
So throw out your food calorie counters. The human body doesn't work like that.
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