I'm guessing the fellow who answered you is roughly correct, as small, privately owned businesses generate much wealth in America. I don't think this is the interesting question though, because I don't believe in the premises of your original comment.
The interesting question is what is the distribution of total wealth in the world, mapped onto the country. Making heat maps of this, mapped onto various other things (inherited versus self made, yalevard versus everyone else, etc) will tell you how the world works. It's obvious to me that the world is run by a narrow self-serving oligarchy, and that we are in the extractive phase of history, such as was experienced by late Rome and 14th century Venice. It's not so obvious how to show this to other people, or what to do about it.
Total U.S. GDP is about $15T. Of that, $8T is compensation of employees, so roughly half accrues to wage-earners (this ranges from the janitor to the CEO, but is probably exhibits a bit less inequality than returns to capital. $5.6T is gross operating surplus: this is profit that accrues to incorporated businesses. GDP is defined as wages + profits + gross mixed income (effectively profits accruing to unincorporated businesses; things like sole proprietors and landlords) + taxes - subsidies. Subsidies is negligible (about $60B) and taxes are about $1T, so that leaves about $500B as GMI (this seems pretty low to me, but I guess most business owners choose to incorporate these days).
Now the question is how much of the $5.6T in profits accrues to publicly traded vs. privately traded companies. This isn't broken out by the government stats, but you can estimate it from financial data. The total market cap of the S&P 500 is about $18.5T:
Market cap divided by P/E gives earnings, by definition, so the total profit accrued by publicly-traded companies is roughly $1T. Similar math done on the Wilshire 5000 index gave the same result. So as a very rough ballpark estimate, the total value-add by privately-traded companies is roughly 5x that of publicly-traded companies.
It would likely be hard to find similar figures for 150 years ago, both because you can't just Google up 150-year-old data, and because 150 years ago the NYSE operated out of coffeehouses and the ticker symbol hadn't yet been invented.
The public data set I used goes only up to 2011. We've grown since, but since the numbers for other components also are c. 2011, it's better to compare them directly. (Actually, the financials are from present-day, so it's not an exact comparison, but if anything the error will be in favor of private companies being a greater portion of the economy.)
