Injections such as restylane, jeuvederm, sculptra, radiesse etc can be injected to make the buttocks larger but because of the large volumes involved it would be cost prohibitive and also do not result in prermanent enlargement. STAY AWAY FROM LIQUID SILICONE INJECTIONS - THEY ARE DANGEROUS AND CAN HAVE LONG TERM SIDE EFFECTS.
Fat Injections: The best surgical option if you do not want buttock implants entails fat transfer (Lipoinjections) involves liposuction aspiration collects fat cells,. Only live healthy cells will survive and so they must be isolated first from the general liposuction harvested fat material.The Fat Transfer Procedure involves harvesting by liposuction with a small cannula, rinsing with sterile saline solution and reinectiing it (lipoinjection or fat transfer) through a very small cannula (fine metal tube - like a needle with a blunt end and side hole) through a needle stick opening. There are many variations such as use of a centrifuge, and using enriched growth cell factors or enriched stem cells but at present I have not found this to be of benefit.
Of MOST IMPORTANCE is the selection of a skilled plastic surgeon who is board certified and has experience in this technique. For example injecting the fat with a large cannula or not distributing the fat in a manner so it is interspersed between healthy native fat cells will result in decreased survival, and injecting it too close to the skin can result in nodules that can be visible