GM currently makes more cars in a week, then TSLA does in a year. And it does so on positive margins.
It's certainly possible for TSLA to get to that point, but it is anything but a sure thing. Why anyone would be surprised that it is one of the most shorted stocks in the world is beyond me. If they fail to grow to GM's size, the shorts will be justified.
To put it in tech terms, it's valuing Mozilla > Apple, because Mozilla sold a million Firefox OS phones. Except that in this imaginary world, Mozilla doesn't even ship a web browser, and keeps having to ask its landlord for an extension on the rent payment due date. Obviously, Firefox OS will sweep the world by storm! Who doesn't like freedom? Surely, the users will switch en masse, and leave the old dinosaurs like Apple and Google broke.