Forbes only knows about public wealth. The Rothschilds in the mid 19th century owned half the wealth of the world, and the contemporary argument is that they have not lost money since then, rather it has grown steadily, and is hidden off the books in countless family controlled foundations as well as secret reserves stashed away before there was the type of public accounting we have today.
Heads of state and CEOs have public wealth. The Rothschilds didn't.
How does the total value of privately traded companies compare to publically traded companies, and how many privately traded companies can the Rothschilds plausibly own without it being public knowledge?
Is it possible to find the total market cap of all privately traded companies? Considering that a private company's wealth isn't public knowledge the only thing we can do is estimate using a tool like bizstats.com.
Macro economics can give you a pretty accurate estimate of the "market cap" of privately traded companies: just work your way down from the Gross National Product by segmenting it into public and privately earned. (You could also look at total tax revenue and calculate from there)
Y'know, I went looking for something to debunk this (rather implausible sounding) theory, so I googled "rothschild snopes". Snopes doesn't actually have anything, but I did find this rather delicious thread from the prisonplanet forums, wherein a bunch of conspiracy nuts speculate about precisely who is funding snopes:
Snopes most definitely is a disinfo website much like wikipedia
Anyway, I certainly can't find any evidence to support the contention that the Rothschilds are significantly wealthier than the wealthiest known people, and I don't believe it's possible to have a trillion dollars in assets while flying underneath the radar.
Families grow exponentially. It's possible that over the years it was spread out to the dozens of great great great grandchildren who are probably so far from the core family, that they don't act as a unified group any more.
The Rothschilds had a tradition of the majority of the family fortune going to the first born male, and the others only received, as they say in the Godfather, "a living".
Heads of state and CEOs have public wealth. The Rothschilds didn't.