Up until March 2007, many people believed Wikipedia's growth to be exponential. Changes in the rate of growth has prompted a re-evaluation of that, and the current belief is that Wikipedia's growth fits a logisitic-linear growth model.
YOUTUBE 7-h1puxbmk4 At 31:51 this keynote digs deep into Wikipedia's growth rate.
Figure 1 wikimedia 
Figure 1 is a log scale graph, where exponential growth is shown as a straight, ascending line. From 2002 to 2006 Wikipedia grows exponentially. But in March 2006, something strange happens: growth of Wikipedia article creation and editing slows down.
Logistic curves are prominent in ecology, where exponential growth is bounded by limited natural resources.
WikiWikiWeb Growth webpage 
The biological analogy is obvious: as Wikipedia grows, the opportunity to make novel contributions shrinks. As the keynoter above notes, this leads to an almost Darwinian result -- shrinking opportunities lead to increased patterns of conflict and dominance, which is what we have seen on Wikipedia in recent years.
The question remains: why are resources limited? Certianly in other domains (scinetific publishing, for example) there are no such limits.
The argument over Kate Middleton's Wedding Dress provides a good example of how scarcity comes about in Wikipedia. Early gaps in coverage are seen as opportunities for expansion; later gaps are seen as proving the existence of some unspoken Wikipedia canon.
Counterexamples: Russian Wikipedia is in an exponential growth pattern to 2010, but at a much smaller factor. Might be useful to think about why. png 