The decision as to what looks good is a personal one, and the doctor and patient may not agree. Nevertheless, the patient's desire is what is driving the conversation, and it is up to the doctor to try to understand what will make you happy. If that is a buttock that is bigger than what that particular doctor feels comfortable with, then you should get several opinions before deciding whether or not to proceed. You can look at many pictures of buttock augmentation on this web site, and you will see a great range from moderate augmentation to augmentations that I personally would not call attractive. Nevertheless, those patients were happy.
One of the advantages of fat grafting augmentation of the buttocks, or BBL, is that it shapes the hips, waist and thighs, and brings out the roundness of the buttock set off by a more shapely torso. It may be that when you discuss this with a plastic surgeon and it is demonstrated that by shaping the waist and hips and upper posterior thighs, your buttock will look larger as these areas look smaller, you may decide you do not really want a larger buttock volume per se, but a buttock that stands out more.
It is also clear that personal preferences for buttock size have some cultural influences, e.g., in Brazil, women may see an attractive buttock as being larger than a woman in Europe or North America, generally speaking, might want. So, getting some professional feedback by visiting several plastic surgeons is a good way to start. You can be assured that in the US you can find a plastic surgeon that will give you a buttock augmentation either with fat or implants that will meet your needs.