Losing Weight After Brazilian Butt Lift?

dear Patient: thanks so much for your question. The fat cells that are transferred to your buttocks react like all the other fat cells in your body, if you gain weight, they will get bigger, if you loose weight they will get smaller.  
Hope this answers your question, remember to only go to Board Certified Plastic Surgeons for a BBL since they are trained for this specific procedure and they are the ones who will give you a safer overall result.
If your Plastic Surgeon is FACS and member of the ASAPS and ASPS is another level of safety as well.
Ron Soltero MD FACS
Board Certified Plastic Surgeon
Member ASPS
Member ASAPS

About 60% of fat from the transfer is retained and the size will change with your weight just as the rest of your body does. In other words, the fat acts like it was naturally there after the recovery process

The question of the perfect weight as it relates to BBL is fast becoming one of the most commonly posted questions in the RealSelf community. While it is true BBL is a weight sensitive procedure (in the sense that you need fat to achieve your result), it is never a good idea to pursue short term changes for surgery.
While there is no absolute number (lbs.) in terms of weight for undergoing liposuction with autologous fat transfer, the optimal candidate has reached a healthy and stable weight (preferably with a BMI less than 30, which is associated with a lower incidence of complications).
Potential candidates should also have adequate fat reserves for transfer. This becomes especially important when one considers the anticipated rate of fat resorption and graft loss. I typically recommened patients aim for at least 500 cc transferred per cheek. This would amount to 1 liter of fat (not lipoaspirate) after processing. Candidates with inadequate fat reserves are likely to be disappointed with their ultimate volume once healing is complete. 
If you have reached a healthy stable weight (preferably with a BMI less than 30), the focus should be on maintaining. I typically do not recommend gaining/losing weight for the sake of surgery alone (weight loss for the sake of your health is a different story). It is important to remember that weight changes post procedure can potentially compromise the results of your buttock augmentation (as transferred fat can change with weight just as it could prior). If you plan on losing weight, I would do so before surgery and plan on maintaining afterwards.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).