Based on this single view photo, you can have significant increase in size. A side view photo would've helped to determine your current posterior projection. You have a lot of fat on your back, waist and hips including the lower posterior back or lumbar area. Your arms are a good source of extra fat as is your abdomen which view you did not post. Part of a successful BBL is also sculpting the surrounding anatomical areas to define the buttock shape and dimension in 3D. If you happy with your surgeon, discuss this further procedure with him/her. Otherwise, seek a second opinion from a board certified plastic surgeon experienced in advanced body sculpting techniques for an in person consultation. Good luck.