Building Pectoral Muscles Before Breast Augmentation?

It will not make a difference in recovery. So you can go ahead and do your weight lifting and be rest assured that it will not make your recovery and more or less painful. What works well during the recovery phase is muscle relaxants and make sure your surgeon provides a prescription for some in addition to the pain medication that you are prescribed.
Good luck.

Most breast augmentations are done by placing the implants under the pec muscles for a variety of reasons, but whether they are bulked up or not should make litle difference in the early recovery. However, if you are a body builder or athletic, there are issues that are more important long-term with implants under the muscle. One common example is what are called "animation deformities" in which the breast is distorted every time the muscle is flexed. A variation of the subfascial technique can help minimize this.