Weight loss after lipo/BBL?

Thanks for the question. In my practice i recommend to have a body mass index 30 or less. For losing weight before a procedure is better to have a consult with a nutritionist to avoid unnecessary risks. Kind regards and good luck. 

In most cases of weight loss, the buttocks and other areas with stored fat will become smaller.  About 50% of the fat that is transferred lives and becomes a part of you.  This will hold stable as long as there is no weight fluctuation.  The fat will not move, but losses in weight will distribute differently.  The fat that lives in the buttocks from the procedure will get larger with weight gain and smaller with weight loss.
Kenneth Hughes, MD, Board Certified Plastic Surgeon
Los Angeles, CA

This question of weight gain/loss 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. 
With regards to your specific question, I have found that patients who lose weight post-operatively potentially compromise their result (if the weight loss manifests in the grafted area). Remember grafted fat will behave just as if it had never been grafted. If you gain weight for transfer, and lose the weight post-op, you are likely to lose your result. Thus maintaining the pre-op weight post op will be the safest with regards to maintaining your results.
That being said, I would not change your weight for the sake of surgery. If you have reached a healthy stable weight (preferably with a BMI less than 30), the focus should be on maintaining.  
As always, discuss your concerns with a board certified plastic surgeon (ABPS).