It is not the GDP and not even production, but the trade consumption and demands that determine the economic strengths, thus affect the debt repayable capabilities. That's why New York replace London as the world trading center in after world war 2.
So in the scale of economy, it is ridiculous to compare Greece with the USA. It is like comparing an infant try to trade their piggy bank with an adult with well establish trade networks.
Thus, unless Greece able to produce products demands by the rest of the world (imagine something like iPhone, not commodities like olive, industrial like tourism, etc) and has a controlling stake in it, otherwise, Greece cannot balloon its debts in the similar level as USA. This rules also apply to resource-rich country. That's why in 2008 world economic crisis, UAE can't simply print money to absorb Dubai debts.
The Missippi Company historical event show bubble will eventually burst even with the support of huge institutional body such as government.