1.5 year post op butt shrinkage, do I need a second BBL?

Losing your fat percentage will cause your buttock to get smaller. Gaining fat should get back some of what you have lost. Without additional fat to inject into your buttocks, I would suggest you consider a non-surgical augmentation with Sculptra. Have a formal consultation to see if you're a candidate. Best wishes, DR. ALDO :)

Ideal candidates are at a stable weight,  in good health , and have excess fat in some parts of the body and too little in other regions.  Gaining weight before the procedure is to no advantage and when you lose weight again the fat cells will shrink just like the donor site fat cells.  Typically, the transferred fat results in a significant increase in volume of the body site being treated.

There is no doubt that weight changes affect BBL results and you are very observant with these changes. Aggressive exercise and fat burning will decrease your body fat and decrease the fat in your buttocks. Weight gain affects all areas, often causing you to lose the definition of your butt. Fat will deposit in your hip and flank area and this causes your buttocks to appear less projected. In general, if you burned lots of fat after BBL, gaining fat again (in healthy or unhealthy ways) will likely not get you back to where you were after surgery. Depending on your goals, additional fat transfer to your buttocks may be necessary to get to where you want to be with buttock size and definition.