Animations.

Given different scales and timing, they will look "off" if the character isn't the right size for it. That said, in Oblivion characters didn't vary enough for this to be an issue, but it's still the main consideration from a technical standpoint.

As someone already pointed out, the difference in size between Bosmer and Altmer is quite important, but they all share the same animations without any issues. So I guess this isn't really a limiting factor?

Maybe I didn't understand what you meant.

You are right, hence i added the disclaimed you didn't quote ;p

Animations are usually the reason though.

While i am excited about the height/weight feature, i really do hope that it will be more than just TALL - muscly/thin, SHORT - muscly/thin, i really hope fat people can be made (ie girth) because it would add a lot to Skyrim as a world with regard to realism and variety. i have my nagging doubts though, because of the armor clip issues, i'm guessing true fat people won't be possible, still it will be a great feature, no more everyone sharing the same physique like oblivion, and that will be a lot of fun to see different shapes of people walking around.

If implemented it’s no reason to not use sliders, clipping is not an issue however might be part of the reasons why they merged greaves and cuirass.