At issue isn't the annual output of Greece, but the total outstanding Greek debt held by various financial institutions (still) in almost critical condition. Having Greece default would require ALL holders of Greek debt to mark down their balance sheets with huge discounts applied to Greek debt - and then require various forced-liquidations and settling of other positions to shore up their balance sheet. Then, as different parts of the financial world go through increased selling pressure, perhaps one medium sized bank goes under....which results in all debt of that bank to be valued with a huge discount as well (similar to Lehman), and then everyone holding that bank's debt marks it down....then add Portugal/Ireland/Spain to the mix, and true financial catastrophe follows.
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