Thank you for your questions and photos. Unfortunately without a examination I can advise you in general terms, however I hope you will find them helpful.
Loosing or gaining weight will always change your body contour.
I suggest you first obtain your goal body weight.
The amount of fat injected depends on:
1) the amount of fat harvested
2) the amount of suitable fat that is able to be purified
3) the “stretchability” of your buttock skin
One can try to gain weight prior to the procedure in hope that the areas you gain weight can be liposuctioned.
Caveat: not all weight gain can not be harvested (some may be deposited on your internal organs or areas that can’t be liposuctioned).
Please make several appointments with experienced Plastic Surgeons who listens to your specific desires and who are not only Certified by the American Board of Plastic Surgery but in addition ideally a member of the American Society for Aesthetic Plastic Surgery (denoting by membership as having met additional criteria and a focus on Cosmetic Plastic Surgery).