If you want a natural method of increasing your breast size, gaining fat is the only real way. That's probably not the answer you want to hear if you want to be trim and fit, but the fact is that breasts are mainly fat. However, your breasts are on top of muscles that you can strengthen to give your breasts a fuller look. So if you want to boost your bust, work your pecs.
Breast Composition
You probably don't want extra fat hanging off your body, unless of course it's in your breasts. Fat is what makes up much of your breasts. Fatty tissue surrounds the breast glands and much of the rest of the breasts. This fat makes up most of the breasts' size and gives breasts their soft consistency. Your breasts don't actually contain muscles, so you can't do any exercises that will increase the size of your breasts.
Pecs
While your breasts don't contain any muscles, they are located on top of the pectoral muscles which you can exercise to gain size. This still won't increase the size of your breasts proper, but Michelle S. Olson, PhD, an exercise science professor at Auburn University told "Fitness," you can increase the size of your bust by working out your pecs. Building your pecs adds size to your chest and gives your breasts a nice lift.
Exercises to Perform
Your pecs aren't muscles you might use much in day-to-day life, so specific strength training exercises are necessary if you want to build them up. Olson recommends performing exercises like pushups, planks and bench presses. Unless you're looking to become a bodybuilder you won't need to work out your chest excessively. You should work out your pecs twice a week, giving at least 48 hours between your workout sessions, according to "Oxygen Magazine."
Other Benefits
When you do strength training exercises for your pecs, you aren't just helping your chest. These exercises help shape the deltoids in your shoulders and the triceps in your arms. This can help create an overall sculpted look for your body. Also, if you're trying to lose weight you'll be happy to know that, according to "Breaking Down Your Metabolism," a publication by the McKinley Health Center, increased muscle mass results in a greater calorie burn even when you're at rest.