The "exposures" look like "total principal value". While the market value of some greek bonds has indeed collapesed by 1/2, that's not the case for other countries. So, while you could have "halved your exposure" to Greece by changing the accounting convention from investment to trading, you can't have reduced your Spanish exposure by 1/2 other than by selling (or failing to roll over maturing) bonds.

Economics is politics by other means