The volume of or fat reflects the sheer amount of calories stored as fat inside our fat cells. When we eat calories which we do not subsequently burn, they are transformed into fat AND stored in our fat cells which, as a result, increase in size. As we burn more calories than we eat, we draw fat from our fat cells causing them to shrink in size.
For this reason, unless you plan to maintain extra weight, it makes NO sense to put on pounds in the hope of having "enough fat" for a Brazilian Butt Lift. Any subsequent loss of weight (= burning more than you eat) WILL result in deflation of ALL fat cells including those which were transferred to the buttocks.
As regards aerobics you are touching on two points; building up your muscles AND consuming calories with intense exercise. While the gluteal muscles can be developed with intense directed exercises the attractive hemispheric full look of the buttocks we admire is made up by a combination of fat AND full muscles. While exercing will get you much of the way there, over exercising may burn more of the fat resulting in a more "athletic" looking than a "fuller" buttock look.
Peter A. Aldea, MD