Brazilian butt lift is a liposuction fat injection. I've done this procedure in thin patients and had success. What you doing with his procedure is changing storage areas for fat if you have excess fat in the back, the flank the abdomen, the arms are the inner thighs this fact can be harvested and put in the buttocks to make flat areas more curvy. You cannot make a large buttocks in inpatient but can change the shape and sometimes you can have more curves. If there's found the waist removing the fat from that area making a narrow waist will make the buttocks look bigger. The other fact that helps is that liposuction is not a weight changing procedure. when you remove fat from the abdomen back her flank when calories are stored they can store in those areas and have to be stored someplace else and usually this is the buttocks. So tummy tucks and liposuction alone can make a buttocks little bit bigger.
In my opinion gaining weight for the surgery does not help. The reason why is because fat cells don't replicate. You born with a certain number of fat cells and keep the same number of fat cells throughout life. So when you gain weight for the surgery then lose weight after the surgery the buttocks will shrink with the weight loss so it doesn't really help you unless you stay at the heavier weight.
From your photo I don't see a lot of extra fat but if there's extra fat in the arms, thighs, back our abdomen fat injection may be an option. the problems with the implant our infection and the fact that it can be adapted to each patient the weight at injection can be adapted to each patient's shape.