Adipose tissue. Unfortunately, yes. Breasts are composed predominately of fatty tissue, and if you lose a considerable amount of weigh, your body fat content will diminish, and you may experience a decrease in breast size. If this is a becomes a major issue, you may consider breast augmentation as a remedy. If you are obese, then losing weight is certainly worth the effort, as you will be healthier with less fat.
Weight. Do monthly brest exam, and annual mammogram. You won't lose your breast if you lose weight. Keep a balance between mind and body. Allow time to implement healthy lifestyle. It is the process that is important, the results will follow. If you do cosmetic procedure for your breast, talk to surgeon, and primary doctors in order to discuss myths and set realistic expectations.
Yes, And depending on your personal situation that might be a blessing, especially if you have some back pain from the weight of heavy breasts. If you lose too much --which you can't help; so be it-breasts are mostly composed of fat. When you lose weight, you lose all over. Therefore, you will lose fat in your breasts too.