My left side was just slightly larger than my right immediately after surgery and there were also some wrinkles/folds on the areas in the middle of my chest. I was told that it was most likely swelling and that it would subside.
It did but it took a little over a year. Now, at two years out, the wrinkles have smoothed and the size discrepancy has fully evened out.
Usually, after surgery, there are going to be size discrepancies from fluid retention/flesh trauma/swelling/etc. and only major discrepancies are a concern (like if your left side was huge, it could be a hematoma).
If a long time has passed and you're still noticing more tissue on one side it could be that your surgeon unevenly removed tissue. A revision (which is usually covered) will be in order if that's the case.