Hit Points can represent a lot more than just weight, and Strength is often a better gauge of mass and bulk. Just look at someone with Body of Air -- they cannot pick anything up, punch you, or normally have an effect on the world, but they slam as well as a normal person! This fixes that. The concept of falling damage seems to be that everyone is affected proportionately, so Hit Points work fine, there.

Dustin Tranberg had a good suggestion. Copy the following table and paste it onto your GM's Screen. It makes slams much easier to deal with.