Many patient have expectation that surgeon can deliver almost any shape and size of buttocks just simply filling it with fat. Unfortunately, anatomical structures (muscles, skin, bones and their relationships) determine postoperative shape and size significantly. This is in particular the case with skin and related underlying scaffold that has certain shape (flat, convex, concave) that can not be changed with simple stretching due to injection of soft material like fat. It may take several surgeries (stages) that will include combination of liposuction and additional fat grafting to shape large buttock into a more desirable shape, unfortunately. You should wait at least 6 months until fat volume and skin stretching is completed and then discuss next stage in buttock shaping with your surgeon. Good luck.