Difficult to know without physical exam. If DIEP lost substantial size in the months after your surgery with an early close match, it is possible that the blood flow to one or the other flaps was insufficient to support its size. If the new breast was smaller to begin with, then the amount of fat you had available was possibly less than anticipated. Sometimes a 3rd flap (such as a TDAP, from the side of the breast/chest wall) can be added to increase the volume. If that isn't an option, then addition of an implant, fat injections to the new breast and adjustment of the other breast are likely the only remaining solutions for better symmetry.
Hope this helps,