Hi
Unfortunately there is no straight forward way to reducing your Breast size. You have to follow a balanced diet and start working out.
First I would recommend you get a hormonal tests done such as
LH,
TSH,
FSH, Progesterone,
Prolactin, Estradiol, DHEA SO4 and routine biochemistry.
This will help you understand if your hormonal balance is okay or not. If that is not okay then you need to consult a good gynaecologist.
But if your tests are okay then there is a high probability that your weight and shape is owing to food habits.
You need to go straight to the gym and start a High intensity workout and get a Nutritionist to write you a balanced moderate carb and good protein and fibre diet.
Health Tips
I wish you all the best.