Does fat end up being absorbed if too much is transfered?

The controversy surrounding the "permanence" or "temporary" nature of the results buttock augmentation via autologous fat grafting speaks to the variable nature of fat grafting as a technique.
Technically speaking the results of fat grafting should be permanent in the sense that grafted fat which has becomevascularized should not disappear several months post op.
However, there is an initial period, immediately post op where some of the grafted fat is lost. It is during this period that some may sense that their aesthetic outcome is fleeting. Every patient reaches a point after which the grafted fat which has become vascularized is permanent and the fat graft which has not will have undergone resorption. This is typically reached by 3-4 months post op. 
For some patients, the graft take vs loss ratio will be favorable and for others less so. There is no question that there is both variability in techniques as well as outcome.
With regards to your specific question, once healed the results are permanent. Prior to that point there is some variability. Implants give a more consistent result. However, the less natural feel makes these options less desirable.
As always, you should seek a board certified plastic surgeon (ABPS) who is experience in both the science and art of autologous fat grafting to help you understand the procedure and provide a reasonable idea of what to expect both immediately post op and long term.

Hello dear!!
Thanks for the question and provided information as well.
After a BBL, only about 70% of the transferred fat cells will survive, so it is normal to see a minor reduction in your buttocks. If you have any concerns, I suggest you to follow up close with your plastic surgeon.
Good luck :)