What happens if you lose weight after a BBL?

The simple answer is...it depends.  Most likely, the fat cells that have been transplanted will shrink in size and so will the overall size of your butt.  But that can be relatively unpredictable.  I hope that helps and wish you the very best!

When you lose weight after BBL, the fat cells will shrink (so the size of the buttocks will decrease). 

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 would not change your weight for the sake of surgery. You should reach a healthy stable weight. I have found that patients who purposely gain weight before surgery will either struggle to lose weight post-op or will lose it in the grafted area post-operatively. Remember if you harvest areas of short term weight gain and transfer to the buttock, this will preferentially be the site of initial weight loss.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).