This graphic comes from Gretchen Morgenson's front-pager in the NYT yesterday. I'm not going to try to reproduce it here, because my column width isn't big enough to really see what's going on. But suffice to say that it shows the market in credit default swaps, at $45.5 trillion, dwarfing the markets in US stocks ($21.9 trillion), mortgage securities ($7.1 trillion), and US Treasuries ($4.4 trillion).

Morgenson's article makes it clear that it's reasonable to directly compare market sizes like this. Indeed, she refers to CDSs as "securities" in the third paragraph of her piece:

The market for these securities is enormous. Since 2000, it has ballooned from $900 billion to more than $45.5 trillion -- roughly twice the size of the entire United States stock market.

But of course a credit default swap is not a security, it's a derivative. The $45.5 trillion is a notional amount; the size of the stock market is a hard valuation. There's an enormous difference.