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.

However, despite the lack of official proof or definition, some researchers have already used the concepts of 'set-point weight.' Additionally, my "base weight" can be confused with "baseline weight," which is used in some studies. Therefore, I will use 'set-point weight' or ‘set-point’ for body weight going forward."

     
In this example, Person A's set-point weight is forty-eight kilograms. However, for Persons B and C, the weights they quickly revert to when they let their guard down and eat a lot—sixty kilograms and eighty-seven kilograms, respectively—can be considered their actual set-point weight. This means that their homeostatic functions are working to maintain those weights. 

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 more likely to be restricting calories and eating modestly, so their current weight is often lower than their set-point weight. On the other hand, thin people don't have caloric restrictions, so their current weight and their set-point weight are often close. 

Therefore, ‘thin A’ won't gain weight even if she eats a lot, and B and C will gain weight immediately as soon as they eat a lot.

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 set-point weight was fast. Those who have tried diets and eat less than usual might have experienced this. Often, it's called the rebound effect.
    

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 lean people－the reason why some people gain weight. 

I believe that each person's ‘set-point’ for body weigh 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