Am I too small for a BBL? I am currently 126 pounds with a BMI of 21.6. (photos)

A good candidate has enough fat available for transfer in the procedure. The amount of fat that can be transferred in a BBL depends on how much fat the patient has to harvest and the amount of space there is to move the fat into. It is recommended for the patient to have a BMI of 30 or less. If you are too thin, a Brazilian Butt Lift may not be possible.

Patients interested in BBL often ask questions of the necessary volume for transfer. Ultimately, the answer is two pronged and based upon both patient expectations and the nature of fat graft themselves.
BBL and fat transfer is ultimately dependent upon the long term viability of the fat grafted. That being said the resorption rates of fat transferred can be significant. Clinically this translates to a progressive loss of grafted volume in the months following the procedure.
I tell patients that they can expect resorption or graft loss on the order of 50% (at most; prepare for the worst and hope for significantly better). Additionally some harvest volume is lost in processing the fat graft prior to transfer. Thus I generally recommend that patients seeking even the most minimal volume increase have adequate reserves for grafting of 500 cc per cheek (at minimum).  This means that harvest will usually exceed 1 liter (assuming for processing loss).
However, it important to remember that these are recommendations and can be amended based upon a patient's particular expectations. Some patients only want a subtle change and may be willing to accept a less significant result. But the rule still applies you have to have the fat to transfer the fat.
Ultimately, there are limits both on donor yield and recipient capacity. There is no defined space for filling thus there is a component of stretch and expansion that goes along with transfer. Once the limit is reached, injected fat will extrude from the injection sites. Overfilling will only lead to greater loss and place you at risk for necrosis.
Based upon your photos, you have minimal donor sites for harvest. You will benefit from the contouring effect of liposuction, irrespective of the yield for fat transfer. I often find that the happiest BBL candidates are those that recognize both the benefits of waistline contouring liposuction as well as the volumizing effect of the fat transfer. I do not, however, believe you have adequate reserves to achieve a significant augmentation as seen in your goal photos. 
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

hello thank you for your question and provided information as well..Based on the photos shown, it would be ideal to consult in person with a certified plastic surgeon who can feel and see if the amount of fat you have is enough to go to surgery.but based on your case of not having enough fat percent you can opt for buttocks implant, which give good projection and beautiful shape.