In a BBL, how much is the average increase of projection (in inches) after injecting 1,000cc's (per cheek) into the buttocks?

There is great variability as the elasticity of the skin will be variable as will the surface area of the buttocks.

Ideal candidates are at a stable weight,  in good health , and have excess fat in some parts of the body and too little in other regions.  Gaining weight before the procedure is to no advantage and when you lose weight again the fat cells will shrink just like the donor site fat cells.  Typically, the transferred fat results in a significant increase in volume of the body site being treated.

Dear formentera1995,it depends on a lot of different factors (anatomy, amount of transferred fat, surgeon's technique, etc.). It is hard to tell without an examination. I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery