If you were to place 5lbs of fat and 5lbs of muscle side by side the fat would look about twice the size of the muscle because muscle tissue is more tightly packed than fat tissue. A 150lb person with toned muscle and a low percentage of body fat will appear slimmer than a 150lb person with low muscle development and a high percentage of body fat. As we get fitter we gain more muscle so the scales may not always accurately show our weight loss. So use other methods (as well as the scales) like a tape measure or the way your clothes fit to determine the amount of fat you've shed from your body.
CLICK IMAGE TO VIEW LARGER