Is it true that muscle weighs more than fat? If so, will I gain weight when I start strength training?

Unless you increase the amount of calories you are eating, it is very unlikely that you will gain weight or become bulky. Here's why: one pound equals one pound regardless of whether the pound is fat, muscle, or some other substance like butter or steel. Muscle is denser and therefore takes up a smaller amount of space per pound than fat. Some scientists estimate that the "space" that one pound of muscle occupies is about 22% less than one pound of fat! If you begin strength training and continue to eat the same number of calories, you may lose some weight because you're burning additional calories while exercising. The important thing about strength training is the change in body composition. You will gain muscle and most likely decrease body fat even if your body weight stays the same. In our experience, people might drop a size or two after they have been strength training for a couple of months because their body shape has changed for the better. If your goal in starting strength training is to gain weight, we recommend you also increase the number of calories you are consuming. Try adding an extra fruit, vegetable, low-fat dairy and/or whole grain serving to your daily diet.