It really shouldn't be so surprising. People get preoccupied with GDP figures, but GDP is a flow, not a stock. The actual value of the United States is (quite obviously) substantially more than GDP. These are approximates, so bear with me.
The asset value of the US is ~$160 trillion dollars, and the liability value is approximately $100 trillion. These assets and liabilities are transitory - people are constantly buying and selling them. Today's asset is tomorrows liability, and vice versa. Whenever these transactions occur, a bookkeeper needs to make the appropriate accounting record. That's DTCC. Given the numbers above, and given transaction and monetary velocities (variable, but let's say about averages about 20x over time), your quickly talking about quadrillions of dollars worth of transactions. Factor in the rest of the world, and all of a sudden its real money.
And this isn't even entirely accurate; there is a great deal of value that is not accurately accounted for in the asset and liability categories, like off-the-books value (handymen, housewives, etcetera), and the exotics like derivatives (technically this is because derivatives have no value on their own; they trade based on the expected value of the underlying asset - but they're marketable). Point here is twofold: economists have a hard time defining "wealth" and tracking it reliably, and the United States is tremendously wealthy (GDP numbers fail to capture the scale of it appropriately).
