Why is one bigger than the other? (Photos)

At 5 days you are about 3 percent healed, settled, shaped, softened, dropped and fluffed. When concerned always best to check in with your surgeon. "Drop and fluff" refers to the fact that as the skin stretches and the muscles relax, the implants move down and out slowly – about 1 percent a day. It’ll be about six weeks before they even begin to settle down, but they’ll be close to 80 percent settled and softened at three months. Watching their progress is like watching your hair grow. You don’t see a difference in your hair growth daily, but all of a sudden you need a haircut. 
Be patient after surgery. Give yourself a few months before making a judgment about your surgical results. You won't know the outcome until then. 
Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author

Asymmetry after breast augmentation is not uncommon. This is temporary, except in cases where there is already an asymmetry before the surgery. Each breast heals at a different rate and one may swell more than the other, thus the asymmetry. It usually takes around 6 months for the swelling to subside and by then you will see your final results. You are still too early int he healing process and your appearance would still undergo significant improvements in the coming months.