Unfortunately, without a picture of your figure, or at minimum your height and weight, there is no way to know what is possible. As far as the Brazilian Butt Lift, there are many different techniques used. Some
surgeons report fat survival rates of 20-50%. Other surgeons expect at minimum 70-80-% fat survival. I have seen some patients of mine who I would estimate have close to 100% fat survival. I have seen these patients in follow up several years after their surgery with little to no change in their size.
In my experience, the thinner you are, the worse the results are with Butt Implants because you have less soft tissue to cover them, even when placed in or under the muscle. Also, when considering the Brazilian Butt Lift, the thinner you are, the less fat there is to transfer. But!!
The thinner you are, the more dramatic the results are even with a small amount of fat injection. Think about it this way.... Adding 300 or 400cc of fat to someone with a big butt (Natural volume of 1500cc) will be a drop in the bucket and you will see little change.
Adding 300 or 400cc of fat to someone with a small buttocks (natural volume of 500cc) the more dramatic the improvement will be.
I have also seen many women who think, or who have been told that they don't have enough fat to have the Brazilian Butt Lift, yet, they still have plenty of fat and get beautiful results.
Be sure to consult Board Certified Plastic Surgeons with significant experience with these procedures.