Is there any way to reduce breast size without undergoing surgery?

Possibly : Breast tissue is primarily fat. Depending on your BMI if you are overweight or obese females tend to lose some breast tissue with weight loss. If you are already a healthy weight then it would be difficult to reduce breast size without breast reduction surgery.

No: A woman's breasts size is genetic, familial, and increases through high school and sometimes for a few years after. Staying slim is likely the only counter measure. Gaining weight adds to the breasts. Being pregnant increases breast sizes too, but they shrink after breastfeeding stops.