How much cc's are in 1 lb of fat? And how do Dr's inject >1000ccs...the math doesn't add up?

The question of the perfect weight as it relates to BBL is fast becoming one of the most commonly posted questions in the RealSelf community. While it is true BBL is a weight sensitive procedure (in the sense that you need fat to achieve your result), it is never a good idea to pursue short term changes for surgery.
While there is no absolute number (lbs.) in terms of weight for undergoing liposuction with autologous fat transfer, the optimal candidate has reached a healthy and stable weight (preferably with a BMI less than 30, which is associated with a lower incidence of complications).
Potential candidates should also have adequate fat reserves for transfer. This becomes especially important when one considers the anticipated rate of fat resorption and graft loss. I typically recommened patients aim for at least 500 cc transferred per cheek. This would amount to 1 liter of fat (not lipoaspirate) after processing. Candidates with inadequate fat reserves are likely to be disappointed with their ultimate volume once healing is complete. 
If you have reached a healthy stable weight (preferably with a BMI less than 30), the focus should be on maintaining. I typically do not recommend gaining/losing weight for the sake of surgery alone (weight loss for the sake of your health is a different story). It is important to remember that weight changes post procedure can potentially compromise the results of your buttock augmentation (as transferred fat can change with weight just as it could prior). If you plan on losing weight, I would do so before surgery and plan on maintaining afterwards.
With regards to your specific question, 1000 cc or 1 liter is equivalent to 2.2 lbs. However, this is not a direct weight correlate. Just because one may have a weight loss goal of 5 lbs does not necessarily mean they have 2 liters or more available for fat transfer. Without photos, it is difficult to provide estimates. However, smaller framed individuals will often times have difficulties reaching transfer goals of  liter per cheek (after processing losses).
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Only your surgeon will know what is reasonable for your body with the liposuction and BBL that the surgeon usually performs.  If you have 2000 cc of fat to be harvested, I would get it.Kenneth Hughes, MDLos Angeles, CA

You have to know that more than 50% of the fat removed from liposuction can not be transfered because of the fluid it comes with it, because of the adiposites being damaged, so if we do  4000 cc of fat being removed, probably only about 2000 or less would be transferred.  More or less, every pound of pure fat (after being treated: decantation and straining) will be about 400-500 cc of fat.To be honest I do not think (based on your BMI) you would have 1000 cc's to transfer. Be encouraged. What ever amount you get even a smaller of what you are expecting (400-600cc's) will enhance beautifully your slender body.Dr. Cardenas