Hmm how does this work with respect to runtime material instances?
For example, if you have 100 doors, all with the same mesh, and with the same unchanging material, your scene will consist of 1 mesh and 1 material.
However, if material parameters on any of the doors can change (textures, color, whatever) then that requires 100 runtime material instances. So your scene effectively has 1 mesh and 100 runtime material instances.
^ i think thats how it works? 
Material instances on disk vs. runtime material instances can get a bit confusing if not specified which.