Well, they've been able to cram a LOT of bot into the alt mode, and vice versa. So they could pull it off so it's not so obvious, but the Beetle won't be 1:1 with the other cars, whatever their scales come out to be. There's no way. I'm assuming from the looks of that pic, that the Beetle will be roughly 80%-85% of the size of a real to-scale Beetle when paired with the other cars.
There's a pic of a Beetle next to a Z car in that thread, and while the Beetle is in the foreground (which throws off the comparison a little) they're roughly the same size, if the Beetle isn't actually stouter/taller.