Greece's and other southern European countries' debt has been considered a problem since the 2008 crash, and austerity measures have been forced to the people.

However the United States also have a colossal debt, yet I have never heard of it being a problem or that the United States requires austerity measures.

I have no idea how accurate they are, but the US debt clock displays a debt with 14 significant numbers in US$, while the Greece debt clock "only" displays a debt with 12 significant numbers (also in US$).

I know that the population of Greece is much less than in the U.S., however I do not think it matters to people "lending" money to the governments - In both cases it seems extremely unlikely that this money will ever be refunded. Also Greece receives more tourists each year than it's own resident population (15 million tourists for 11 million residents) so it's hard to compare.