Looking at a Windows Vista or Windows Server 2008 installation is “why is the WinSxS folder so big?!” To answer that question I need to first describe componentization, and how components are managed in Windows Vista.
One of the largest changes between previous versions of Windows and Windows Vista was a move from an INF described OS to componentization. A component in Windows is one or more binaries, a catalog file, and an XML file that describes everything about how the files should be installed. From associated registry keys and services to what kind security permissions the files should have. Components are grouped into logical units, and these units are used to build the different Windows editions.