Those games are far far more compact than Skyrim. Skyrim is a big game that uses a lot of processing, those other games aren't a sprawling as Skyrim is and I don't think they have as much to do as Skyrim.
So why can people create mods that make the animation better and the models better and it will still run well? O.o
Oh yeah, add some graphics mod on top of that for overall world and you still run it fine.
Creating characters with better animations, better models doesn't take a lot of processing while playing, that's foolish. Clutter, shadow effects, textures on ground, tree's, walls, etc - that take up processing beyond way more than creating non-stiff-claw-hands-models.