China is a very distant 4th place when it comes to spam. You want to know who leads the world in spam output; its the wealthy EU countries followed very closely by Japan-Korea and the US. I don't know where that 20% for china comes from. From a study done in March of 2005:

1) Europe(*) 24.70
2) Japan-Korea 24.24
3) US 22.80
4) Greater China(**) 14.45

(*) European Union countries: 21.85%; Top spam-distributors: French, Spain, Germany, UK
** Including: China, Taiwan, Hong Kong.

source: http://www.clickz.com/stats/sectors/email/article. php/3491796/

Of course, I also see numbers like this from a slightly older article:

"Sophos, Inc., an anti-virus and anti-spam company based in Lynnfield, Mass., reports that the U.S. -- sending out 42.53 percent of all spam -- sits far atop its list of the world's Top 12 Spam-Producing Countries."

So, just depends on who you ask on how it breaks down, however, either way, it isn't China.