Talking to my mate who works in the Wolfsburg plant as an auto electrician, he claims it's because in the early nineties, when the T4's were first screwed together, this was the standard for the sizing of a 12v socket. There were no mobile phone chargers or GPS chargers back then, so manufacturers just chose a socket off the peg without a thought to standardising them to the cigarette lighter size. There is no reduced load as far as I'm aware.
I've got a handy lead with a twist off cap, with the cap on, it fits a standard 12v socket, with it off it fits the tiny socket in the back.