One boob is a lot bigger than the other. Is this normal?

No, a "massive difference" is not normal; on the contrary, it may be a sign of a complication such as fluid accumulation.  This should be ruled out by your plastic surgeon;  follow-up with him/her for accurate assessment, advice and/or meaningful reassurance. Best wishes.

At 3 weeks, you are about 15 percent healed, settled, shaped, softened, dropped and fluffed. When concerned, always a good idea to consult 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

Thank you for your question. It is not uncommon to have swelling. Breasts may not heal at the same rate, causing one breast to appear different than the other. It can take up to 12 weeks for the implants to settle and you to see the final result. Without photos it is hard to give a more definitive answer if you may have a complication. Follow up with your board certified surgeon to address your concern. Good luck!