My site was 4 in December. I had a lot of pages but many of them were hidden. So I redid the navigation then pr lowered to 3. But good thing is that it's holding very steady right now. Many of my pages are indexed after the redesign. No, there was no drop in links. I read in a blog that sudden increase of large quantity of pages will lower the overall pr because the votings that the homepage has to give to the internal pages. Is it true?
PR points are basically distributed. If your receive backlinks pointing to your domain page only, the PR of it could be lowered after the addition of inner pages. But if this happened, you probably must have added 50 + of pages suddenly. That's just my guess though. I actually don't know any case that losing domain PR because of a sudden increase of inner pages.
PageRank has absolutely nothing to do with the number of pages you have. Noting to do with whats on the page, on the site, the color of the moon, or anything other than the links pointing to a page.
It has got something to do with how many links there are on that page, if he links to more pages from his site the PR is diluted.
Before the PR was concentrated on the pages you are now missing PR from. The PR is now more evenly spread due to the better link structure.