After a breast reduction, is it possible that's what cleared up my acne? (photos)

I doubt the breast reduction at any effect on the acne directly. However, the perioperative use of antibiotics may have improved your complexion.

I would say that the most likely cause for the improvement is the antibiotics that you have been taking for the breast reduction. You may want to discuss this with a dermatologist to control your acne even in future.

Congratulations on two great outcomes: smaller breast and clearer skin!  It's likely that the antibiotics you were given around the time of your surgery cleared up your acne.  Your breast tissue doesn't create enough estrogen to have a real effect on your skin.  If your acne flares up again, see a dermatologist and let her know what antibiotic you were on during your breast reduction recovery.