wingslevel - 2:49 am on Jun 1, 2012 (gmt 0)
I have been looking at this for years too. I have had a suspicion that some of my sites are throttled. If you look at my google referred traffic, it makes an amazingly consistent picture when looked at over months. Of course, over the last year we have had sitewide separate algos in the form of panda and penguin, so as backdraft pointed out, they tend to trump anything. So our discussion here should try to be net of their effects.
What I am thinking these days is just that the consistency comes from the relationship of off page to on page ranking factors. Off page rules. So, each site gets so much trust weight - skipping for now how google calculates trust - think of page rank as a very dumbed down little peak at this. Imagine a 1,000 page site with home page pr of 3. It might well have just it's home page at pr3, 50 pages at pr 2 and 150 pages at pr 1 - the rest get the white bar pr 0. So 80% of the site might have negligible trust juice - I think these pages, no matter how quality the content and on page optimization, are going to be relegated to the lower tiers of googles index and they will rarely get impression.
In many sites, I think the trust ranking is in fact the throttle. If the above site generated 1,000 more pages of really good content, I think it would get the same number of visitors from google organic search - unless those pages generated good inbound links and other trust signals that would raise that site's overall trust score.
The shift in balance toward off page factors favors the big brands because they have huge trust scores.