The amount of fat is important. However RESHAPING the buttocks is more that just injecting fat.
It involves reshaping the buttock by removing fat in the flanks and illiac crest, sometimes in the posterior thigh, when indicated.
Then harvesting all the fat needed and injecting the fat into the areas of the buttockss that give the best roundness, projection and curves
Brazilian Butt Lift - the more Tummy fat the better Up to a Point...
Every woman is individual with a different buttock anatomy and different volumetric fat grafting and shaping needs. It is nice to have such a fat volume available on one location without a longer procedure involving getting it from multiple sites. But - you must remember that the primary operation is butt enhancement with fat sculpting NOT abdominal sculpting of an obese patient. In most states no more than 5 liters of fat may be liposuctioned in one outpatient procedure.
There more fat one has the more that can be harvested and injected in the gluteal area. So yes the more abdominal fat the more that can be injected for a brazilian butt lift.
Liposuction plays a big role in the gluteal contouring, so this is also a factor, meaning the contouring that is done around the lower back, hip and lateral thigh area.
I have noticed that there is more buzz at the Plastic Surgery meetings of the abundance of stem cells in abdomenal fat. This may be the best resource you have to harvest fat for transfer to fill areas you wish to augment.