There is Lotte World in Seoul, South Korea. Billed as the world's largest indoor amusement park. I've seen references to being in the Guinness Book of World Records, although I can't find the exact figures.
I've been there, and while it is pretty big, I found it hard to believe that it was the largest in the world. Maybe I was still salty at the fact that I got kicked out of line for Atlantis for being "too tall".
http://www.lotteworld.com/Global_eng/Main.asp