They stretched for height, so why not weight as well, plus the body sizes did vary a slight bit in Oblivion and those meshes stretched to fit.
1. The game wouldn't run. That would be too demanding. No developers do this.
2. They would need to cut such a useless and ludicrously demanding feature in favor of something that will actually run and is tangible.
3. The developers responsible should sign a life-lasting contract forbidding them from ever optimizing software again.
That doesn't mean important data can't update during that time, just because it isn't rendering.
OP: Yeah I don't see how the issue of clipping is a reason why they'd merge greaves and cuirass. This isn't some new feature to Skyrim that's never existed before separate meshes for armor pieces. Also greaves and cuirasses are two large portions of armor that it'd be stupid to merge them together to have one piece only. It made sense for the pauldrons to be merged with the cuirass as it covers such a small area and some cuirasses may already naturally cover that area anyway, but to make all armor sets jumpsuits, that right there just does not make any sense. Yeah they had that in Fallout 3, but again, that was Fallout 3 a different game and a game where you didn't spend as much time in close combat as there was far more ranged combat going on and so armor didn't matter as much, just like real life. As soon as firearms became used in a massive amount, armor became less useful as most of the firearms can just go right through the armor. In our current time, we still do use armor but it's only militaries and law enforcement who pretty much stick with uniforms so no need for customization there. Sorry to get a bit off track with my examples, but I'm sure some of you guys can see my point.