Body weight depends on many factors. A few pounds more or less don't decide whether a person is slim or fat. The important thing is that the weight should lie within the normal boundaries. Body Mass Index shows the relationship between body size and weight, though it tells us nothing about fat as compared to muscle mass.
It should be remembered that BMI is only suitable for people of average physical build. BMI cannot be applied to people who have a high proportion of muscle mass (e.g. body builders), as it does not distinguish between fat and muscle. Also for children and young people in the growth this categorization of the BMI cannot be applied.