I ask this question because I have been on a low-calorie diet and have been working out on my total gym once or twice a day and walking 2-2.5 miles a day five days a week for the past two weeks. In the mirror, I can clearly see muscle definition and fat loss, but my weight has only decreased by but a few pounds and, some days, it has actually gone up a pound. I am aware that muscle is heavier than fat and that muscle has a tendency to retain water. I also made it a point to say "all things being equal" because I am aware that how fast muscle builds vs. how fast fat burns depends on the intensity of your workout and also your diet. However, if my muscle tissue is building faster than I am burning fat, that would introduce an additional factor that could explain the erratic numbers I see while I am on the scale.