Slim more refers to losing weight or other reasons, making the body very thin, giving people a feeling of looking weak, fragile and even depressed. It emphasizes physical weakness and helplessness, making it easier for people to communicate with patients or extremely weak people.
Slim is a broad description, which can be used to describe people with slim figure and elegant lines, as well as some parts of the body, such as hands, feet and arms. It has no obvious emotional color and will not remind people of weakness or helplessness.
Generally speaking, although these two words are similar, their specific meanings and emphases are still different.