What do you mean you have about size 18 breasts? Like what an 18 year old should have? Although that is really unpredictable...
Anyway, if you are overweight, if you wanted to lose weight & get more fit, when you began to exercise, you could possibly lose some of the fat in your breast... *probably not a lot, though*
You should really consider being happy with what you have and making the most out of it, but if they're really big and effect your daily life and self-esteem, you could consider plastic surgery.