First of all, I want to explain the most important point. It's the assumption that each person has the ability to maintain their present condition, also known as preserving current stasis in the body.
I recognize that this is the precondition of everyone regarding weight control.

All through the year, we get thinner when we are busy, and we gain a little fat when we aren’t active and eat a lot. Although everyone repeats such things, even if we don't calculate calories strictly, the body shape of person won't change so easily. Fat people are fat and thin people are thin.

So, here, I want to define the term “base weight” as the weight to which you go back when you continue eating a lot for three to five days doing nothing such as during the long holidays. Usually in most cases, you won't go beyond the base weight and will usually come back to it.
  

In this example, the base weight of A is forty-eight kilograms(48kg), that of B is sixty kilograms(60kg) and that of C is ninety kilograms(90kg). There is the mechanism to maintain their present condition working in their body to bring them back to their base weight.

So, it’s difficult to assume a person’s body and weight condition only with caloric intake.

Consider the example above, that if A continues an intake of 100kcal over her recommended daily caloric intake every day for several months or years, the assumption is that it will accumulate into fat and she will eventually gain weight up to the eighty-kilogram level. This is wrong (She might gain weight but that is a different mechanism).

In general, people who are overweight are living with caloric restrictions, so their present weight is lower than their base weight.

On the other hand, thin people don't have caloric restrictions, so their present weight and their base weight are often close. Therefore, thin A won't gain weight even if she eats a lot, and B and C will gain weight as soon as they eat.

But, as soon as the match was over and he started to eat, his weight increased ten kilograms in a few days. The rate of going back to his base weight was fast. Those who have tried diets and eat less than usual might have experienced this. Often, it's called the rebound effect.
    

I arrived at this idea of "base weight" through my own experience and by observing people around me, but according to Dr. Jason Fung, the author of “The Obesity Code,” in 1984, two researchers, Keesey and Corbett, first proposed that there is a "set point" for body weight and fatness. 

According to this idea, homeostatic mechanisms in the human body work to defend this body set weight against changes. 

However, I have a different view about it.

It makes total sense that when the body has less energy available, it lowers its metabolism to conserve energy expenditure, and that when you eat a lot, your basal metabolism conversely increases, but isn't that a consequence of what happens in the body when you increase or decrease your intake of calories? That idea may explain the rebound effect after dieting, but it does not explain the fundamental difference between fat and thin people－the reason why some people gain weight. 

I believe that each person's base weight (set weight) value is determined by the absorption ability of the   intestines (the small intestine is also known as the "second brain”). This is my conclusion from my own experience, and I believe I can prove that theory with the help of experts.
    

References：
[1] Dr. Jason Fung, The Obesity Code, 2016, Page 62