^yep earthquake height limits. Basically Tokyo has the biggest buildings in the world in terms of average floorspace, but they are all squat and hovering around the 4-500 ft mark, and 6-700ft mark above that.
Anywhere else such as NYC and Hong Kong and these buildings would be half as wide and twice as tall. If you read between the lines Tokyo would have most skyscrapers in the world if it wasn't for the zoning:
http://building-pc.cocolog-nifty.com
www.imageshack.us
for example, the Ropponggi Hills complex has almost the same floorspace
as the Sears Tower, although it looks like this:
www.wikimedia.org