First, a quick introduction to a truth that I'm israel email list not sure Google's shared very publicly (though they may have discussed it on panels or formally on the web somewhere I haven't seen) - that is - the concept that there's an "indexation cap" on the number of URLs from a website that Google will maintain in their main index. I was skeptical about this until I heard it firsthand from a Googler being described to a webmaster. Even then, I didn't feel like the principle was "confirmed," but after talking to a lot of SEOs working at very large companies, some of whom have more direct interactions with the search quality team, this is, apparently, a common point of discussion and something Google's been more open about recently.
The "indexation cap" makes sense, particularly as the web is growing exponentially in size every few years, often due to the production of spam and more legitimate, but no less index-worthy content on sites of all sizes and shapes. I believe that many site owners started noticing that the more pages they produced, even with very little "unique" content, the more