Fat transfer, if done properly by the fat transfer surgeon it ensures survival of the transferred fat in its new location. The recipient area does swell up a bit as a part of healing process, thus it may appear to be over-corrected or over-filled for 10-14 days. The swelling will generally subside by 2-4 weeks after the procedure. In some cases, approximately 10-20% of the transferred fat may get absorbed as a part of the healing process. This occurs over the initial 2-3 months.
Generally, the transferred fat that survives (80-90%) beyond this stage will survive and continue as a normal part of the body. However, if you lose or gain significant amounts of weight after your procedure then the transferred fat would also respond by corresponding reduction or enhancement in volume. Fat transfer done for breast augmentation or buttock surgery may not be significantly altered with minor weight fluctuations. However, fat transfer done to the face may show noticeable disproportion as the transferred fat (body fat) grows more rapidly with weight gain than facial fat.
Thus for long lasting results with fat transfer, it is advisable to maintain your body weight after the procedure. Minor weight fluctuations (2-5 kgs) may not alter the result significantly but larger weight fluctuations (5-10 kgs and above) will certainly show up as noticeable changes or disproportion.