Firstly, not all patients will be suitable for breast augmentation with fat grafting. And I'm not talking about not having enough fat. Almost everyone will have some fat they would like to donate to their breasts.
When I perform cosmetic surgery for the breast, it's not purely about size and having bigger breasts, if that were the case, then, yes, everybody can have fat injected into their breasts. The best candidate for this procedure should not have any droopiness of their breasts. If fat is injected into droopy breasts, you will end up with big droopy breasts.
Another important factor in the decision process is what kind of result you want. Do you want a very full upper pole with a lot of cleavage or do you want a more natural result? Fat grafting is great, but it can never give you the full cleavage that implants can give you no matter how many times you repeat the procedure.
There really is no maximum cup size that can be achieved with fat grafting (as long as you have enough fat), but I wouldn't recommend going higher than a D cup. Anything larger than that using your own tissue will cause sagging of your breasts in the long term.
I have performed fat grafting on very slim patients with excellent, natural, scarless results. I would suggest an in person consultation to evaluate whether a patient is a suitable candidate for fat grafting.
Hope this helps.
