The term "natural weight" as used here is misleading. It does not mean that people are supposed to be a particular weight. It refers to the idea that something—whether it's diet, genetics, environmental factors, or something else—influences the body's weight regulation mechanisms to "prefer" a certain body weight, and changing that is very, very hard.

Think of it like body temperature: normally, your body maintains a certain core temperature, and it will respond in a negative feedback loop when the temperature deviates from that set point. Conditions like infections can induce where your body raises that set point to fight the infection, and it takes time for that set point to come back down.

Something similar appears to happen with people's weight. Hormonal or other changes cause the body to gain weight, and once that happens, it resists losing weight in various ways: increasing appetite, changes to basal metabolic rate, changes to metabolic pathways involved in breaking down nutrients from food, all sorts of things. Deviously, fat cells actually produce hormones that play into these changes, which is one of many reasons why losing weight is usually harder than gaining it. And when people do manage to lose weight, often after a huge exertion of effort and willpower and major lifestyle changes, their set point may not have changed, and so as soon as they slip up even a little bit, they quickly regain back to their set point.

So they're not suggesting that there's some mystical force out there that decides some percentage of the population should be overweight. They're referring to the idea of a set point for body weight that is difficult to change without drugs or bariatric surgery.