Mac OS X is a bloated operating system. When an operating system wants to support different architectures and different languages all in the same software package, on the same disk, it has no other choice to duplicate resources for different CPU architectures and different languages. It cannot do magic so it takes up your valuable disk space (it's specially valuable if you're using a MacBook Air with a Boot Camp partition, not to mention the 64GB SSD version).
Fortunately, there's a solution for this problem that will effectively free up your disk space by removing unnecessary languages, keyboard layouts and binary code from you applications and the operating system.
An open source project named "Monolingual." does the task painlessly and reliably. I haven't had any problems with it so far.