I found some original size headlamps with the plastic faces, but the filament base or back side of the bulb was glass or something and they weighed more or less the same as the old school glass types. I'm banking that I was wrong and just gave up too soon. Perhaps someone can jimmy in with an example and have real world weight differences and part numbers and pics.
At this point, I don't have the time to research with real items, but perhaps later on.