If I gain weight for a BBL will my face remain bigger after the procedure?

I typically do not recommend weight gain for BBL due to issues such as this. One of the benefits of fat transfer operations is that it will forever alter the distribution of fat in a favorable way. About 50% of the fat that is transferred lives and becomes a part of you.  This will hold stable as long as there is no weight fluctuation.  The fat will not move, but gains in weight will distribute differently.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

When you gain weight for a BBL, the goal is that you keep that weight on after the surgery. Otherwise your booty will just shrink away with the weight loss. That means if you gain weight in your face or neck, those areas will remain so in order to keep your booty the way it looks post op. 

Unlesss you are prepared and dedicated to keeping your ENTIRE body about 20+ pounds overweight for eternity that "gaining weight for a BBL" is a bad idea.  I know because I have at least three patients come to me per week after failed (ie melted away) BBL by this exact technique. Please read on for what I typically tell potential patients like yourself:Unfortunately, because ~80+ % of the fat transferred will melt away within a year, most patients are not good candidates because they lack an adequate amount of fat to harvest. Beware, if a surgeon asks you to purposely gain weight (i.e. fat) before BBL, know that the fat you lose first as you get back to your baseline weight after surgery is that very same fat that was transferred into your buttock, hence your new buttock shrinks first, fast, and the most...so don't fall victim to this recommendation. Even those patients that had adequate amounts of fat pre-operatively, still end up seeking buttock implants after a year or so because most of the fat transferred melted away leaving them with minimal result. 
 Although overall using your own fat is relatively safe, it not infrequently melts away unevenly leaving one butt cheek bigger than the other or with dimpling or hard fat cysts.  The one serious complication that can rarely (< 1%) happen is "fat embolism" in which some of the fat gets into the blood stream and travels up into the lungs, heart, and/or brain causing serious problems. This complication is more likely to happen with the larger amount of fat being transferred. This is even more likely to happen when using fillers like liquid silicone, PMMA, Sculptra, and hyaluronic acids. Also fillers, when injected in large quantities, have a relatively high infection rate, guaranteed tendency to migrate away from the original area they were injected, and almost always stimulate a lot of inflammation with a subsequent and disastrous amount of scar tissue/hardening.
Thus buttock/hip implants are a relatively very good, safe, and long term reliable option for most patients seeking buttock/hip augmentation.  Contrary to nieve claims by many other plastic surgeons, intended to scare patients away from buttock implants, my overall complication rate is only ~5%.