the body composition is used to determine just that, how much of your body relatively is fat. Its not necessarily related to weight because it doesn't determine how much fat is there, just how much is there comparatively. I personally believe that the body composition ratio is a more accurate test than weight because there a plenty of health people out there that are skinny and plenty that are heavy with muscle, but if your naturally skinny and are heavy due to a high amount of fat there are numerous health risks involved with that.