Its not thinner. The x240 is 0.8 inches thick [1] and the MacBook Pro is 0.71 inches thick. [2] Not trying to fanboy here having used both of those laptops recently.
Thanks, and sorry about the misinformation. I got my info from http://www.apple.com/macbook-pro/specs/, which was the first result for a search for "Macbook Pro dimensions". Didn't realize that was apparently obsolete info.