BBL for small frame? I'm 5'3 and125 lbs, but it's so difficult to put on weight

You do not need to gain weight to get an excellent result if you select the right surgeon.
Kenneth Hughes, MD

Thanks for your question. Without any photos, its hard to make an assessment just based on your weight whether or not you would be a good candidate for BBL.  During this procedure, fat is harvested via liposuction from different areas of the body such as waist, back, flanks and thighs and then transferred to the buttocks. I recommend an in-person consultation with a board certified plastic surgeon who can evaluate how much fat can be removed and transfered to your buttocks at your current size. They will then be able to give you a better idea of what the expected results would be.

Hello dear!!
Thanks for the question.
I would recommend you to post pictures to give you the best recommendation possible. If you are healthy it is possible to perform the procedure.
 I recommend to book a consultation with a board certified plastic surgeon in your area to discuss the best options for your goals. 
Good luck :)