Hi Jesse - in the .3dm file, MoI sets the units to a special "no unit system" value. It appears that Rhino doesn't have a way to set this in its UI, but it is available to programs writing a .3dm file.

When Rhino imports a .3dm file with this setting in it, it seems to just bring the values over into the currently defined unit system with no changes (with no unit conversion message). So for instance 3.5 will come in as 3.5 mm if your Rhino units are mm.

So it works for you because you are considering MoI's generic units to be millimeters, and your setting in Rhino is set to millimeters too, so they match up.

- Michael