it's a cheat of sorts the wire is shown form viewport 2 so you're seeing the viewport smoothing, hence the less dense mesh.
I create a new layer with all the objects in then i make everything including the background black (alt +b) then i do one playblast with the master layer the shaded models then i do one playblast with the second layer set to wireframe on shaded with every model shaded black. This one i can overlay the wire on top of the first playblast (or render works with both) it's a very quick way to get a wire overlay.
Hope that makes sense if not i'll be happy to do a short vid.
Dave
From a readers' Q and A column in TV GUIDE: "If we get involved in a nuclear war, would the electromagnetic pulses from exploding bombs damage my videotapes?"