Aaron Swartz, who is running for the WIkipedia executive, has done some data-crunching using a rented supercomputing cluster, against many Wikipedia entries to determine how Wikipedia entries get written. It turns out that while the majority of edits come from a small group of 500 core editors, the majority of new content is inserted by drive-by, unregistered users whose contributions are then massaged into encolopediahood by the core 500.
When you put it all together, the story become clear: an outsider makes one edit to add a chunk of information, then insiders make several edits tweaking and reformatting it. In addition, insiders rack up thousands of edits doing things like changing the name of a category across the entire site — the kind of thing only insiders deeply care about. As a result, insiders account for the vast majority of the edits. But it's the outsiders who provide nearly all of the content.
And when you think about it, this makes perfect sense. Writing an encyclopedia is hard. To do anywhere near a decent job, you have to know a great deal of information about an incredibly wide variety of subjects. Writing so much text is difficult, but doing all the background research seems impossible.
On the other hand, everyone has a bunch of obscure things that, for one reason or another, they've come to know well. So they share them, clicking the edit link and adding a paragraph or two to Wikipedia. At the same time, a small number of people have become particularly involved in Wikipedia itself, learning its policies and special syntax, and spending their time tweaking the contributions of everybody else.
Other encyclopedias work similarly, just on a much smaller scale: a large group of people write articles on topics they know well, while a small staff formats them into a single work. This second group is clearly very important — it's thanks to them encyclopedias have a consistent look and tone — but it's a severe exaggeration to say that they wrote the encyclopedia. One imagines the people running Britannica worry more about their contributors than their formatters.