A key part of the Bretton Woods system is the use of scrooge mcduck's reserves of foreign exchange to stabilize the world economy against deflationary pressures. It's unhelpful to think of scrooge mcduck as a hoarder of private wealth. He is, in effect, central banker to central banks, and provider of limitless liquidity.

Like most great fortunes, Scrooge McDuck's was built on luck, hard work and many resources. Since the GDP doesn't take into account new wealth that is undeclared, it can be assumed that McDuck has not declared any of his vast amounts of it, in particular, the swimming pool in his estate filled with gold coins.

As for how Scrooge McDuck got that rich, here's a quote from an encyclopedia article about him:

Scrooge has worked his way up the financial ladder from humble immigrant roots. As a young boy, he took up a job polishing and shining boots in his native Glasgow. His turning point was when a ditchdigger paid him with an 1875 US dime, which was useless as currency in 19th century Glasgow. Enraged, Scrooge vowed to never be taken advantage of again. He takes a position as cabin boy on a Clyde cattle ship to the United States to make his fortune at the age of 13. In 1898, after many adventures he finally ends up in Klondike, where he finds a golden rock the size of a goose's egg. By the following year he had made his first $1,000,000 and bought the deed for Killmule Hill from Casey Coot, the son of Clinton Coot and grandson of Cornelius Coot. He finally ends up in Duckburg in 1902. After some dramatic events where he faces both the Beagle Boys and president Roosevelt and his "Rough Riders" at the same time, he tears down the rest of the old fort Duckburg and builds his famous Money Bin at the site.

Obviously, that clearly can't be the whole story, and it isn't. While Scrooge McDuck did make most of his money on his own, McDuck was never actually hurting for money. He was the heir to the Scrooge fortune of the London Scrooges, the wealthy money-lenders. The fortune itself made Ebenezer Scrooge the richest duck in all of London until he had a nervous collapse on Christmas 1843, at which point his nephew Fred took over the fortune. Fifty years later, it was still sizeable enough that the Scrooge fortune could have conceivably fit into a swimming pool of money.

So to answer the question, tax fraud. The IRS never went after him, and Scrooge never had to care about how he spent his money, but at no time did he spend enough to destabilize the market, except the one time that he decided to simplify his portfolio into more gold by selling his stocks in late 1929.

There are a few leading explanations for Scrooge McDuck's utterly exorbitant wealth. To rather quickly summarize them:

The first (and most dominant) is that that figure you quoted was actually mistranslated, exaggerated, or otherwise mistaken. After all, McDuck was (and is) known for being a bit of a blowhard at times, and his Scottish accent has confused inteviewers on occasion. Additionally, we must consider that McDuck was known to be in a heated competition with Flintheart Glomgold to be the richest duck in the world. As Glomgold was very well-known for being "ambitious, ruthless, and merciless", along with being very cunning and clever, it is not very far-fetched that Glomgold would've overestimated his assets through various means. McDuck, upon learning of this overestimation (the two did engage in various types of corporate espionage-see Glomgold's employment of the Beagle Boys), likely then exaggerated his own fortune, so that he could remain the world's richest duck. Glomgold's various arrests, audits, and criminal investigations caused his fortune to be properly counted, which would have given Scrooge a chance to quietly de-exaggerate his wealth, and thus avoid being investigated himself. This de-exaggeration also explains why the financial system did not collapse: McDuck never had that much money in the first place.

The second theory is that, in that quote, McDuck was not referring to American dollars. A leading hypothesis is that McDuck was actually referring to Brutopian dollars (which, it should be noted, the Brutopian currency is the Brutopian peso, but McDuck likely did not care for the distinction), of which the exchange rate at the time of the quote is (somehow, surprisingly) unfortunately lost. McDuck is known to have had commercial relations with the Brutopian Dictatorship, so the link is definitely there. Alternative hypotheses think that McDuck was referring to other currencies; a rather poorly-regarded hypothesis is that McDuck was actually referring to some sort of extraterrestrial currency, as Duckburg was known for its space-exploration program.

A third theory is, and sometimes followed by a few who align with radical schools of thought, is that McDuck is actually part of a the standard conspiracy theory of some sort of "New World Order" conspiracy that controls the world. McDuck, thanks to his immense wealth, is said to be the treasurer of this organization. It is questionable that, if such an organization did exist, why a single man would hold all of its assets. This theory further states that several other well-known figures, of both the past and present (like sorceress Magica de Spell, various members of the McDuck clan, and even inventor Gyro Gearloose), are also members of this "New World Order" conspiracy. This conspiracy is known by its supporters as the "M.M. Clubhouse". It is unknown what the "M.M." stands for, as the supporters of this theory have not clarified on the issue.

Actually, Scrooge McDuck's hoarding of his fortune has left the duck economy in shambles. Ducks are starving, looking for food and work anywhere they can. A more famous example of this is Howard the Duck coming out of retirement for his cameo appearance in Guardians of the Galaxy. He took that gig for pennies on the dollar of what he made in the 80's but needed the work to support his kids and growing alcohol problem. In fact many many poor ducks have fallen into alcoholism to take the edge off of their awful lives.

Lucky for us, the human and duck economies exists mostly independent, save for sale of bright green Mountain Dew Dale Calls here and there.

The situation has gotten so bad that in the past few years America has started a financial aid program to help the duck population. Proceeds from the sale of Duck Dynasty merchandise goes directly to help revitalize the duck economy. So the next time you are in Walmart and see this useless crap please think of the ducks and buy something, even though you won't ever use it.

There have been some radical new theories on the purpose and hoarding of wealth not only by McDuck, but also his contemporaneous tycoon Richard "Richie" Rich. A recent work -- perhaps Apocryphal -- from the Gallifreyan Matrix Archives that tends to posit the causative factor of most problems due to a renegade Time Lord as "the Doctor", has offered an intriguing (if understandably implausible) solution to this conundrum.

According to this source, the Earth has been the subject of multiple invasions of various different extraterrestrial life forms over the years. One of these were the unimaginitively-named cyborg race the Cybermen, and the other ahive mind called the Nestene Consciousness that apparently inhabits different types of plastics, creating what appears to be mobile mannequins called Autons.

According to this already dubious claim, the Matrix divulged that after seeing multiple failure states having occurred by said "Doctor," these two foes "joined forces" and created an expeditionary force that contained teh ruthlessness of said Cyber creatures but instead of their standard alloyed bodies these forces were made of ultralight advanced plastics, allowing teh Nestene Consciousness to offer superior adaptability to the force.

As you can see, this is already a desperately thinly stretched yearn, but the claim gets even more bizarre. This Matrix source claims that these Cyber-men had some form of inherent reactionary problem to the metallic compound of gold, and that in some manner that involved sonic waves and "reversing the polarity of the neutron flow" (A ridiculous claim, after all, since neutrons have no charge to reverse!) the plastic was altered molecularly to become gold, thereby ejecting the consciousness while rending the cyber creatures inert golden statues.

And this is where this fantastically bizaree story intersects with teh McDucks. Apparently this "Doctor" had a triplet of "Companions" who recognized if said gold was dumpe don teh marketteh Earth's economy would collapse, and they reccomended divinding it between two individuals they knew who had peculair mental illnesses. Their Uncle Scrooge, who has a fetisization for base umping into precious metals; and Richard Rich, who apparently fancied himself some form of archetect who could only work playgrounds and other ridiculouslymundane items out of precious metals.

By dividing up this hoard of gold between the two, the money was taken out of circulation, therefore SAVING the world economy from rapid devaluation. Further, by claiming to their Uncle that if he spent any major substance of his wealth then Richard would become the wealthiest figure on the planet, McDuck was driven to keep his hoard mostly intact.

While all of this clearly seems a ludicrous myth, one cannot deny that Richard Rich's newest theme park at his private estate features a prominent Police Box made of Solid Lapis Lazuli with a Ruby cut dome. But perhaps this is merely coincidence.

I am cross-posting a comment I made a few months ago about a similar topic. I include my primary sources on the document. But it is clear by the evidence in the primary sources that S. McDuck having control of so much hard currency does not have an overall negative effect on the economy (though to say it has a positive effect is not proven).

Scrooge McDuck did a lecture with guest speakers Huey, Dewy and Lewy about how he circulates his wealth by investing and re-investing.

He is an active player in the local and national economy, even if he is a miser of sorts. So he is clearly a boon to the economy by not only providing a ready source of capital to help growth, but even teaching others about how currency works.

McDuck earned most of his wealth through mining. Thus, it can be said that he hasn't taken money out of the economy by stashing all his gold - rather, he literally dug the gold out of the ground and then has not sold it all for fear of destabilizing the economy by flooding it with gold, which would cause a depreciation of gold. This would also cause deflation as McDuck would be sitting on all this cash he would get from selling gold, thus taking money out of the money supply. Instead of doing all of that, he keeps the gold and instead runs an actual business empire to stimulate the economy.

How Glombold fits into all of this, I'm not sure. Some sources say it's from diamond mining.

Its important to consider this question is economic as well as historical terms. A majority of Mr. McDuck's wealth was not in any way active in the market. By keeping over 90% of his wealth in untraded gold coins he was able to maintain the market value of gold, and ensure the (relative) stability of the world economy. Remember that Mr. McDuck grew up in the depression era, and a businessmen of his savvy would clearly understand the role he had in preventing another such decline.