"Smaller, Faster, Smarter"
Every successful software once reaches a stage where adding more features makes the application 'Bigger, Slower and Lamer". Once that critical mass has been reached, trying to cash in by adding more features leads to a process called 'feature creep'(http://en.wikipedia.org/wiki/Feature_creep).
What Microsoft and Apple are trying to do with their latest OS releases (Windows 7 and Snow Leopard) is to offer an upgrade which claims to free up more disk space (smaller), run faster (more optimized, efficient).
Read http://www.nytimes.com/2009/08/27/technology/personaltech/27pogue.html?pagewanted=1&_r=3&ref=technology .