No, you're right, Chinese is the most common spoken language, well, Mandarin. It's the biggest country and the primary language, so..
But, Ethnolouge is basically a census of language, so to speak. They do a LOT of research, and a lot of people work on it. They use percents eligible, try to calculate 2nd languages spoken as percentages of population..etc. It's VERY interesting stuff.
By the way, Kanji is a not a very good indicator, since it is also the basis for Japanese writing, and I can tell you as a Japanese speaker, it is a VERY different language. And that would consider Cantonese too, which is also a very different language than mandarin.
http://www.ethnologue.com/
Feel free to browse all of this stuff, I can spend hours at a time on here!
...plus it takes my mind off the fact my N900 won't be here for a million years.