BBL for skinny petite Asian girl: Can I get a BBL without gaining weight? (Photos)

Thank you for your question and posting appropriate pictures. You sum to be a good candidate for BBL. I do not believe in gaining wait before BBL unless you are going to keep the wait in after. I like to maximize fat take in skinny BBL patients by using WAL body jet. This increases fat take from 60-80%. You would need a consult for precise recommendations either in person or via Skype/FaceTime. 

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.
With regards to your specific question, you appear to have moderate reserves available for transfer. The best candidates are healthy and have reached a healthy stable weight (preferably with a BMI < 30). I don't typically encourage weight gain (unless you plan to maintain that weight post op) as the transferred volume can be lost.
Ultimately, "enough" is relative to expectations. Those seeking dramatic changes need to have the available donor fat. However, your donor fat do not determine your candidacy for surgery. Your candidacy is defined by your expectations. As long as your goals are in line with what can be realistically achieved, then you can proceed.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).