In the most general sense, any activity that gets your body moving is going to help you burn calories, which could help you lose fat if you achieve a calorie deficit, and that could include doing yoga to reduce the size of your breasts. Breast tissue is primarily fatty tissue, so the way to reduce excess fat is by following the same process you'd follow to lose fat anywhere else on the body. In terms of exercise choice, however, you'll have to spent a lot of time on the yoga mat to lose a significant amount of weight.
To lose fat, you have to do calorie-burning exercise. If yoga motivates you and is the only type of exercise you can stand doing, by all means keep doing it. Consider this, however: the standard Hatha yoga class will help a 155-pound person burn an average of 149 calories in a 30-minute session. That's the same amount you'd burn in 30-minutes by walking at a fairly slow pace of 4 miles per hour and only about half as many calories as you'd burn by jogging at a moderate 12-mile-per-hour pace in 30 minutes. In terms of calorie burn, then, it would take a lot of yoga sessions to burn off the 3,500 calories that represents a 1-pound reduction in total body fat.
Not All Classes Are the Same
When it comes to the intensity of yoga, not all classes are the same. In a study sponsored by the American Council on Exercise, researchers looked at the aerobic effects of a standard Hatha class versus a "power" class meant to be more of an aerobic-style workout. In the Hatha class, participants experienced increases in strength and flexibility and were particularly successful in building strength in the chest and abdominals. During the Hatha class, participants burned about 144 calories in 50 minutes, while participants in the power class burned an average of 237 calories in 50 minutes. The Hatha class didn't raise participants' heart rates to a level that is considered "aerobic" exercise -- a level of at least 60 percent of participants' maximum heart rates. The bottom line is that certain forms of yoga may be better for burning calories and therefore helping with fat burning, though most also tend to be good for improving flexibility and strength.
Achieving Ideal Intensity
If you want to use yoga as a calorie-burning exercise, look for classes that tend to move fast and elevate your heart rate significantly. These might include power yoga, Ashtanga and Vinyasa, or "flow" classes. During these classes, instructors tend to move more quickly from one pose, or "asana," to the next, which tends to elevate the heart rate. Also, try gauging intensity by checking your heart rate in the middle of the workout. If your heart is beating at 60 percent of your maximum heart rate or higher, you're in the optimum training zone for cardiovascular fitness and fat burning. The quickest way to calculate your maximum heart rate is to subtract your age from 220, though this is not always accurate for all people.
Strengthening the Muscles
While you may get in a "cardio" workout during yoga, its main purpose is to realign and strengthen the body, to regain balance and flexibility and to quiet the mind. You can't "spot reduce" fat away by only doing exercises that work a certain body part, but if you're concerned about the appearance of your chest area, try placing special focus on that area through targeted exercises that can tighten the core and chest muscles and reveal healthy muscle tone. The classic Sun Salutation sequence includes planks, cat/cows and upward-facing dogs -- all which can help to tone the chest muscles. To stretch the pectorals, also include the upward-facing plank and half-frog poses into your regular routine.