So, you're saying that if you are equally warm in a dense undergarment as you are in a poofy undergarment, then there is the same amount of air inside the suit?
I know you were asking tbone, but I think the answer is yes. The fibers take up space. A suit is poofier because the fibers take up more volume. The more high-tech fibers are thin, or even hollow, and so take up less volume. There is better insulation because more volume is occupied by air and less volume is occupied by fiber.