In a case like yours, to get a more hourglass shape, you need not just fat injected in your buttock. You need to have a tummy tuck with the liposuction, so the excess of skin on the abdomen can be removed and the rectus muscle on your abdomen can be repaired. The you will have a smaller waist ...
Thank you for posting the picture, you seem to have realistic expectations and that is the first step for a successful cosmetic procedure. You can discuss with your PS to go as big as possible ( depending on how much fat can be harvested from your body , in case you are thinking of going bigger...