Sorry, what I meant was if you look at the squares that they use to give the scale of the map, in the bottom RH corner, they have made the sides of the square proportionate to GDP. So, if you look at the square corresponding to $500B, and the square corresponding to £1000B, the $1000B square is in fact four times the area of the $500B square, not twice its area.

I think it might be an error in the scale, not the map, though, because by eye the US does look to be about nine times bigger than, say, france, which is what the numbers also suggest.