Your body has a limited capacity to digest both protein and fats, which are passed in the excrement if you exceed that capacity. Carbohydrates, on the other hand, are digested primarily in the stomach from which they are transferred directly to the blood stream, elevating your blood sugar. Your pancreas creates insulin in response to elevated blood sugar which is a hormone that instructs your cells to switch to sugar metabolism. This shuts down fat utilization in the body as your fat cells stop participating in the gluconeogenesis cycle and begin converting sugar into fat. Only after your blood sugar returns to a normal level can your body switch back to fat metabolism and begin the gluconeogenesis process again and start eliminating body fat. A diet that is nearly entirely devoid of carbohydrates, as nearly everyone's diets were prior to the rise of agriculture on the order of 8 to 12 thousand years ago (which isn't that long ago from an evolutionary perspective) forces your body to run primarily on fat metabolism and combined with the aforementioned capacity limits on protein and fat digestion limits your maximum weight. Previously in human evolution carbohydrates were limited by availability and seasonality. (Good luck finding fruit in the winter.) But the rise of agriculture, industrial agriculture, global trade, and factory-produced processed foods has lead to a scenario where the vast majority of humans find it more convenient to eat their food from a boxed product - consisting primarily of carbohydrates, which again halt fat metabolism and tell your body to begin storing fat instead - and are consequently gaining weight on a pandemic scale. I'm happy to provide references and more information if you like.