Don't get me wrong, if I'm looking for the Apple website, I should absolutely get Apple.com first in Google, and not a medium-sized blog talking about Apple. So relevancy should matter most.
However, I fear that they are ranking bigger sites and bigger brands above more relevant posts from medium-sized, sites, too. And that I don't like. Give the little guy a chance, especially if his post is of higher quality.
And no I don't think that if the big site's post gets retweeted 100 times and the medium-sized' post gets retweeted 10 times, it should matter much, because resharing is just a side-effect of being big and having a big audience, and I think it's less about the "quality" of the post.
Google ends up promoting bigger and bigger sites at the top, while downranking the smaller ones, who are impacted negatively by a lot of factors (smaller age, fewer backlinks, fewer reshares, etc).
So I guess my point is, on-page "performance" (for lack of a better word) should always count more than off-page performance.