How much fat do I need for a BBL to see a difference?

Without a side view photo or knowing your goals, I am unable to specify how much fat needs to be harvested and transferred to your buttocks. Gaining weight is often counterproductive. If you gain weight that is fat, some of the fat will likely be associated with the buttocks.  In almost every case, I do not recommend weight gain prior to a BBL. I think it is better to maintain your stable weight. 

Buttock shaping/augmentation is a combination of liposculpting of waist, lower back, thighs and selective augmentation with fat grafts. A significant improvement can be achieved in many patients with just taking away fat deposits from flanks and lower back without augmentation; buttocks look larger, more prominent; visually it looks like augmentation. Adding fat grafts (volume) to some portions of buttocks (usually upper and lateral-hip) will additionally shape buttocks and make them more harmonious with back and thighs. Amount of fat grafts needed to achieve this effect will depend of individual patients desire and anatomical structure (bones, muscles, fat distribution). You seem to be good candidate for BBL, in person consultation is suggested. Good luck.