A person may weigh more or less depending on the bones besides the amount of fats or muscles that are in the body. There are several other factors besides height that influence weight. Moreover, a person may have quite some weight, but that weight may not be due to fat. Let us take the instance of a couch potato and a bodybuilder. The former may be leading a sedentary lifestyle, but his BMI may within the desirable range, and yet she/he may have superfluous fat tissue. On the other hand the bodybuilder will have heavy weight (BMI) which may not be an indicator of fat. The high BMI is due to the high percentage of muscle tissue in his body.
It is to be noted that the muscle tissue weighs more than fat. Too much fat leads to overweight which uncared for degenerates into obesity. Excess muscles compared to fats may increase the weight of the body as muscles tissues weigh more than fats. But that person may not suffer from health complicacies as she/he would have had there been an excess accumulation of fats: hence the need for excess muscles but limited fats.