1. Characteristics of lean body mass
In fact, combined with my friends, many of my friends are the kind of thin physique that can't gain weight after eating too much. They don't have to worry about gaining weight at all. Because they don't gain weight if they eat too much, I feel that their stomachs are often much more than those of fat people. They are especially able to eat and eat a lot, but they digest quickly and defecate on time every day. Eliminate toxins and feces from the body.
2. Easy-to-lean physique depends on personal preferences.
In fact, the fitness of being thin mainly depends on whether you personally like to be thin. For example, these thin friends around me all want to gain weight. So these questions vary from person to person. But personally, I think it's better to be thinner. When you are old, you don't have to worry about fatty liver or three high schools. Generally speaking, it is still relatively healthy.
In short, I hope you can have an opinion after reading my above answer. You can have an objective idea about your weight gain or loss. But I want to remind you that you should never get fat until you are old. It is easy to cause disease.