Brazilian Butt Lifts are just a liposuction procedure with the suctioned fat being transferred to the buttock. Liposuction is not done for weight loss but for sculpting and shaping the areas of concern. In general your weight will not change after liposuction unless a very large volume of fat is suctioned (multiple liters). Even when large volumes of fat are suctioned it is rare for a patient's weight to drop more than a pound or two.
Your weight should remain very much the same if you simply do a fat transfer to your buttocks. If you have a liposuction and some of the fat is not transferred but simply discarded you may weigh somewhat less. My Best, Dr C