Gerry McGovern has written an article on the impact of adding more content to a website:

Organizations are now spending more and more on getting people to their websites. However, just because someone visits your website doesn’t mean you have reached them. Your website may be too cluttered, with too many messages to too many audiences.

Every time you target a particular audience, you make another audience less visible and important. If you are trying to talk to 10 audiences on your homepage, you might as well be talking to none. As readers quickly scan your page, they see lots of messages that are not for them. Impatient scan readers like to be made feel special. Otherwise, they hit the Back button.

Every time you add a piece of content you make it more difficult for another piece of content to be easily found. Sure, you only make it a little more difficult. However, as you keep adding content, the navigation becomes less intuitive, and the search less effective. (Most people will not go to the second page of search results.)

» Gerry McGovern: Web manager: you can't serve everybody