As someone who knows nothing about cars, I'm trying to figure out why a glass roof would necessarily result in more headroom and can't find any more info. I've

As someone who knows nothing about cars, I'm trying to figure out why a glass roof would necessarily result in more headroom and can't find any more info. I've found other references to this stated as a given, but with no explanation. Surely you could just build a "normal roof" at the same height as a glass roof, right? Are normal roofs more than 3-4 inches thick and glass can be much thinner? Surely, say, sheet metal could be thinner and lighter than glass. Or is there some reason why it enables a different cab geometry with a greater height? I'm stumped.