How does Stack Overflow serve 60 million pageviews with 5 servers while Digg requires 500 servers to serve 200 million page views?
There is a great difference in load when serving different types of content. Static, text based pages are very light, whereas active pages can impose a heavy load on a server.
The underlying data structure also plays a big part - using efficient techniques to load data from optimised databases for display can make a huge difference. Have a look at the ServerFault blog, where various aspects are discussed.