Radiance-general Digest, Vol 56, Issue 6

I've come across the same thing in the past and thought that it was due to the model being a set of NURBS surfaces that rhino is segmenting to turn into a DXF. I think there is a way to try and get rhino to clean some of this up but haven't found a fast process to do so.

Yep, often Rhino produces horrible large meshes for solid / brep objects.
If you simple select all objects in your scene and run the "explode" command, then if you mesh your objects the meshes are really just fine.
Best thing is to make layers of material, than mesh per material layer.

Writing a rhino -> radiance mesh converter with RhinoScript is pretty trivial too, if you need to do this ask more often or are interested in developing a workflow.