Volume isn’t the reason they’re measuring. It’s only part of the overall procedure.
It’s plain to see you’re incapable of making any progress.
As I said, the doctor isn’t interested in the volume but rather the percentage of body fat, again, it’s not like the doctor can carve the fat off of you and measure that separately and distinctly from muscle, bone, etc.
It isn't the "final cause" fer sure, but it is the reason they are immersing people in water rather than simply weighing them on a scale.