The two sides of the human body are not perfectly symmetrical, they always differ from each other. Usually this difference is slight, but sometimes it can be significant. These facts are true for women's breasts. Mostly the difference is minimal; the patient may not notice it if attention has not been called to it. However, in the case of surgery we should let the patient know this, because the difference will possibly exist after surgery, when the patient may be more critical and consider it to be a result of the surgery.
The difference of the breasts exists because of the difference in shape of the chest bone. It also can be the result of curvature of the spine. It is also possible have almost a 50% difference in volume and shape. For this we can get the required result by an implant (if the difference is in volume), or with mastoplexy, or with a combined technique of the two. You can read about the process of the surgery and the possible complications in the chapters dealing with these questions.