It wouldn't take that. Just some animation. I'm not saying that it should actually be physically THERE. It should look like it is though. And that would NOT take a lot of harddrive or a supercomputer to run.
For it to "look like" it's there, the system has to calculate and animate it. Even if (like the entire
imaginary game world) it isn't "physically THERE." It would actually take some doing, and the whole skin wrinkling/stretching thing when you stand or sit is something you
might notice if...you were playing in third person and stayed zoomed in on the flexing body part you wanted to watch the stretch/wrinkle of. In normal play, the odds of noticing it are pretty slim indeed. As in, "much ado about nothing."