In the search ecosystem Google controls the relevancy algorithms (& the biases baked into those) as well as the display of advertisements and the presentation of content. They also control (or restrict) the flow of marketable data. For example, a publisher might not get keyword referral data on organic search, but Google passes that data on via advertisements & passes a large amount of data on through their ad network to other ad networks. Consider this:
a DoubleClick tag on the site sent data to two other companies that collect it for various purposes -- Rubicon and Casale Media, representing a "hop." In a subsequent hop, Casale transferred the IMDB data to BlueKai, Optimax and Brandscreen, while Rubicon pushed it to TargusInfo, RocketFuel, Platform 161, Efficient Frontier and the AMP Platform. AMP then sent the data on to AppNexus and back to DoubleClick.
For about a decade being relevant & focused created efficiencies that more than offset any "size = quality" biases that the Google engineers created. However across many verticals that window is closing & it is never a good idea to wait until it is fully closed to adjust.
This shift from relevancy to "size = quality" can be seen in the stock performance of mid-market companies like BankRate & Quinstreet.

Those companies were laser focused on the markets that have significant consumer intent & traffic value, but Google has eroded the affiliate base & ad networks of many of the direct marketing plays for a couple years straight now.
If Google's algorithmic biases are strong enough to literally move the market on companies worth hundreds of millions to billions of Dollars, one is naive to swim against the tide. The market is becoming more bifurcated.

This is why it is so hard to find a great SEO to recommend for small businesses. If that SEO really knows what they are doing & understands the market dynamics, then they probably won't serve the small business end of the market very long, or if they do, they will do so in a way where their continued flow of payments is not tied to performance. It is hard to have a sustainable business operating in a closed ecosystem if you are swimming in the opposite direction of that ecosystem.
We're Going Google... | SEO Book.com

Pretty clear that google have radically changed!