On Mac OS X (I'm using 10.8/Mountain Lion), you can inspect files in the Finder via Option-Click -> "Get Info". Doing so opens a panel with all sorts of information. In the top right corner, you can see the file size in MB. Next to "Size" (filed unter "General"), you get more detailed information: The size in bytes, as well as another size in MB on disk.
I noticed that some files have a substantial difference between their size in bytes/the value in the top right corner, and the on disk value. One example is the Clang binary:
Can anyone explain why there's such a huge difference (26.3MB vs. 10.9MB)? Are parts of the file shared (I have installed the Xcode command line tools, so the file exists in multiple locations)?