The "meat" that we pay more attention to is actually composed of fat, muscle, bone and so-called "water". Some people look fat, but their weight is not heavy, because their meat is mostly fat and water, and the meat is loose, so such people are more likely to lose weight (except pathological). You said that some people "look thin, but their weight is not too heavy", which may be because their meat is short of fat and water, mostly muscle, which is often called "porcelain solid". Part of the reason may be that he has a high bone density and a large skeleton. So you look thin but not heavy.

Of course, it is not excluded that these people are good at matching clothes and modifying their bodies, resulting in the visual effect of "looking extremely thin but not light in weight".