There are several reasons someone may want to reduce the size of his thigh muscles. For example, a professional body builder may want to lose mass in order to transition into fitness competitions--which have different body shape criteria. Whatever the reason, there are ways to safely reduce size in your thigh muscles, but keep in mind that it is not possible to spot reduce. In the process of losing muscle in the thighs, you may also lose muscle in other parts of the body. Also, the legs have some of the largest muscles in the body and it may take a while to see any progress--you may notice shrinkage in other muscles, such as the biceps, before you notice a change in the thigh muscles.
Change your resistance workouts. Muscles naturally shrink, or atrophy, when you stop using them. While you cannot stop using your legs entirely, you can stop or alter leg-specific resistance exercise.
If you work your legs several days a week, reduce the frequency to once or twice a week. Use lower weights with higher repetitions, which builds endurance without bulk. Alternately, you can stop all leg-specific resistance training and focus solely on upper-body conditioning.
Increase your cardiovascular workout. Try non-weight bearing exercises, such as cycling, swimming or the elliptical trainer. Cardiovascular exercise breaks down muscle tissue because the body burns sugar first, then breaks down fat and muscle for energy. Exercise for 45 to 60 minutes most days of the week, and replace at least one of your leg-specific resistance workouts with a cardio session.
Alter your diet. Muscle is active tissue and burns calories at all times. Reduce your daily calorie intake and your body will break down muscle to compensate. Reduce by no more than 500 calories a day and follow a healthy diet with fresh fruits and vegetables, whole grains and lean proteins. Do not go lower than 1,200 calories a day and eat at regular intervals, otherwise your body will go into "starvation mode" and hold on to fat to compensate for the loss of calories.