doc_z - 4:02 pm on Jun 26, 2012 (gmt 0)
I guess that Google is not measuring the quality of a page/site by the bounce rate.
As already said there are many examples of pages with high bounce rate but good content (e.g. searches for phone numbers, zip codes, lottery numbers). On the other hand clicking on more than one page on the same website doesn't mean that this is a high quality site, because it might be caused by the fact that the information is buried.
I guess that Google is measuring the quality by "the user is still searching on Google for the same phrase after leaving the website" rate (continue searching rate). (You can see that they pay attention to this point when you hit the "back" button and get "Block all www.example.com results".)
The reason why this method would prefer big brands is the following: People know what they get on these well-know websites. Therefore, people who dislike these results don't click on the search result. This leads to a lower continue searching rate (which indicates a better page/site) for well-know websites compared to an unknown websites.
Of course, just a theory.