Fully agree with small = more DV and big = less DV, and I strongly wish to see that implemented. It would help balancing the game without having to change the damage modifiers too much, and therefore without dimming the differences between small and large creatures which are IMHO one of the best new features of ADOM II.
About speed, I don't agree. Yes, it's true that bigger legs can get you more speed, but that's about the only speed-related advantage a big creature gets over a small one (in real life). For the rest, as you say, bigger creatures are much slower to react (imagine an elephant vs. a mouse), so just giving large creatures more speed would be very unrealistic, apart from unbalancing. As someone said in other part of the forum, a small guy with a rapier should get a lot of small hits on a giant before he is able to hit him with a large tree trunk.
So I think the realistic thing to do with speed is that larger creatures have *less* speed, but movement for them requires fewer energy points than for smaller creatures (something like the 7LB's effect).
Actually, for balance, I think it would be good that large creatures ran a little bit slower than normal creatuers anyway, regardless of realism.