Asymmetrical breasts 1 year post op - What could have caused this? (photo)

Breast asymmetry after breast augmentation can have multiple causes. Breasts are almost always different to begin with. Even breasts that appear naturally perfectly symmetric are not if you look carefully. Natural breast asymmetry results from differences in soft tissue volume and volume distribution, differences in nipple position, differences in skin characteristics, and differences in chest wall shape (ribs and pec muscles).
Surgical technique and implant selection can can sometimes improve symmetry if the reason for natural asymmetry is mild. However, even with volumetric 3D imaging to help with planning, it is very difficult to consistently correct asymmetry because there are so many causes of asymmetry and implants aren't necessarily made with a given individual's differences in mind.
Breast augmentation can make asymmetry worse if the surgeon makes a mistake in implant selection or creates a space that is not perfectly positioned and sized for the implant they use.
To help figure out why your breasts are different I need to see your standardized preoperative photos, review your operative report, and see standardized post operative photos including from above with you laying down to see how much your implants fall to the side.
Final thoughts: your photo suggests you have an excellent overall result. Yes, your breasts are different, but the differences appear to be fall well within the normal range.

Only on the basis of in person evsaluation, a surgeon can find possible reason for asymmetry. Since you did not post your preoperative photos, it is possible that a small asymmetry was present before surgery but your surgeon did not address that issue with you. It would be in your best interest to see your surgeon and discuss your problem. Good luck.