The Windows SxS directory represents the “installation and servicing state” of all system components. But in reality it doesn’t actually consume as much disk space as it appears…
In practice, nearly every file in the WinSxS directory is a “hard link” to the physical files elsewhere on the system—meaning that the files are not actually in this directory.
The WinSxS directory also enables offline servicing, and makes Windows Vista “safe for imaging”.
While it’s true that WinSxS does consume some disk space by simply existing, and there are a number of metadata files, folders, manifests, and catalogs in it, it’s significantly smaller than reported. The actual amount of storage consumed varies, but on a typical system it is about 400MB.
http://aspoc.net/archives/2008/11/20/winsxs-disk-space-and-windows-7/
