why is your underwear relevant to anyone on Yahoo? If your really concerned about your chest you should be discussing it with a physician. Beyond your underwear, there are certain "abnormal" conditions that can cause a man to have breasts. However, since your still in puberty I would be more concerned about why you feel the need to wear a bra. I'm assuming your parents aren't aware of this. I'm also assuming you like wearing a bra. If these assumptions are correct, then perhaps you like cross dressing, or are just experimenting which is OK ! But...you should explore this crossdressing issue with a professional. As your sexual identity progresses, and as you mature, you want to make sure that your head doesn't get messed up, There are support groups that support cross dressing in most larger cities, and also groups that help adolescents explore their sexuality. If one of those groups are not available to you then you should visit a GLBT web site and maybe call a hotline for some free liteature so you can better understand where you are going with this.
If my assumptions are wrong, and your breast development is physiological then yes you need a physician because you may need some therapeutic medicinal treatment You didn't give your height but your weight is probably high for your age and height so it may just be an accumulation of fatty tissue around your breasts. You might try diet and exercise, especially to the upper body. If you don't have dumbells, just do some lifts with some cans of vegtables, just a few pounds and 50 sets each arm will increase your pecs to where it will burn off some of that booby fat. Check out online sights for the best way to excersize your pectoral muscles. Since your only 15, muscle development is imprtant for the rest of your adolescence alot of that might go away.
Since you have large breasts, you probab;y take alot of ribbing from your peers in gym class etc. You know what...your body is your temple, you must treat it with respect, it is where you will live the rest of your life. Dont make fun of yourself or let anyone else make fun of you for being different. I find your whole bra question on here to be self depricating, that means it is demeaning to you...I'm quite sure that is not what you intended. If this really bothers you, get it checked out by a physician if there is no medical issue, then by all means start some meaningful excersize and sculpt your body into one that you don't have to make fun of...either that, or live with the fact that is who you are...but in the end you have to be comfortable with how you look or your life will be miserable. Good luck.