Does your butt grow after having a Brazilian butt lift if you gain weight or work out?

Transferred fat behaves just as if it had ever been moved. It will (potentially) grow with weight gain and shrink with weight loss. Gaining or losing weight can compromise your result, thus the ideal patient has reached a healthy stable weight pre-op.As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Successful fat grafts may be considered to be revascularized and theoretically are metabolically active, meaning these cells can make more fat if there are more calories to be converted to fat storage.  

Gains in body fat can be distributed to the buttocks and gluteal muscle can hypertrophy with strength training.

Brallen,
As you gain weight, the fat transfer to your buttocks will also increase.  With working out, you usually burn fat which should decrease the size of your buttocks unless you are doing squats or other glueteal exercises which will maintain or enlarge the buttocks.
Dr T

If you gain weight after a BBL the fat that has been transferred to your butt should get bigger too. Most people would not recommend enhancing your buttocks through unnecessary weight gain, but to answer your questions, the transferred fat cells to your buttocks should respond like they would have before they were transferred and get bigger or smaller with your overall weight gain.I hope this helps!

Yes, your buttocks can enlarge with weight gain after a BBL just like your body would expand with weight gain before a BBL.