Is the U.S. like Greece?

Demonstrators stand in front of Greek Parliament in Athens during a protest on Wednesday against plans for new austerity measures

NEW YORK (CNNMoney) -- Deficit hawks often cite Greece's debt nightmare as a cautionary tale for the United States.

But is the United States really like Greece?

The differences between the countries, after all, give the United States some built-in advantages when it comes to managing its debt.

The U.S. economy? Gargantuan. Greece's economy? Tiny.

Plus, the United States' earning capacity is greater since it has a more diverse economy than Greece does, said Barry Anderson, who used to run a budgeting and public expenditures division at the Organization for Economic Cooperation and Development.

The pain of Greece's crisis

There are also big differences in the countries' currencies.

The dollar is the world's reserve currency and is managed by a single government.

The euro, by contrast, is used by 17 sovereign countries. Therefore, Greece's monetary policy is set not according to Greece's needs but by the needs of stronger economies -- especially Germany -- that prefer higher interest rates as a bulwark against inflation.(See correction below.)

The OECD, meanwhile, estimates that U.S. net debt will reach 75% of the country's economy this year. That number includes federal, state and local debt but excludes debt owed to government trust funds such as Social Security.

Greece's debt, by contrast, will be around 125%.

So what's the concern for the United States exactly?

"The real fundamental similarity is trend. Both [countries' fiscal situations] are bad and getting worse," Anderson said.

That's because both the United States and Greece have aging populations and mature economies that aren't likely to grow as much as they have in the past.

Simply assuming that the United States will remain forever immune from the perils of too much debt is not a winning strategy, deficit hawks warn.

Of course, that's a hard case to make when investors embrace Treasuries every time they get spooked by turmoil in Europe. Events in Greece in recent days have driven the 10-year Treasury yield below 3%.

In other words, the United States continues to get the benefit of being a "safe haven" compared to the rest of the world.

But experts believe that investors won't always give Americans the benefit of the doubt.

"The markets do react late, but when they react they react pretty sharply," said Carlos Cottarelli, director of fiscal affairs at the International Monetary Fund, during a Committee for a Responsible Federal Budget conference this week.

A little over a year ago, investors' fears that Greece would ever default were considerably lower than they are today. So it's not surprising that the yield on 10-year Greek bonds is now pushing 18%, up from 6% at the start of 2010, according to Bloomberg markets data.

But even if U.S. rates don't rise soon -- and some say they won't because investors fear slower economic growth -- that doesn't mean markets can't find other ways to hurt the United States, Anderson said.

Investors might choose to punish the dollar if they start to believe Congress doesn't have the political will to do what's required to put the U.S. budget on a more sustainable track.

And a substantially weaker dollar would have all sorts of negative repercussions, including a possible credit downgrade from ratings agencies, said Michael Pond, managing director of fixed income strategy at Barclays Capital, at the CRFB conference.

The bottom line: The U.S. is not Greece in many ways. But it's also not immune from the punishing effects of debt.

-- Fortune.com's Colin Barr contributed to this report.

Correction: The original version of this article incorrectly stated the number of countries that use the euro.