Matt Cutts has a very long post that gives details about some webmasters that have problems with BigDaddy update.

"First, I believe the crawl/index team certainly has enough machines to do its job, and we definitely aren’t dropping documents because we’re “out of space.” The second point is that we continue to listen to webmaster feedback to improve our search. [...]

People have been asking for more details on “pages dropping from the index” so I thought I’d write down a brain dump of everything I knew about, to have it all in one place."

http://www.mattcutts.com/blog/indexing-timeline/