The SEC investigated them a long time ago and determined that their trades were legitimate but just done in convoluted ways. There is a recent NYT piece that covers it. There's a new book about Bridgewater coming out so that's why there are so many articles about them recently.
There's no reason BW needs to be a whale.
No matter how big these funds are, liquid global markets are gigantic.
They are in equities, rates, fx, all sorts of derivatives thereof.
They are generally making millions and millions of smaller trades.
So sure they have $100B AUM and maybe 5x levered to $500B.
But across a million positions that's .. an average $500K position size.
Few if any of these funds ever have like $10B sitting in a single trade.
And even if they did, consider the result of a quick google search "Average daily trading volume of U.S. treasury securities 2000-2018. In 2018, the average total volume of treasury securities traded per day was over 547 billion U.S. dollars."
Is it all a front or a Madoff situation?