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In my opinion, the graph in the image above only shows part of the needed data, what I mean is, there's no indication about who uses which phone.
Usually phones with small screens are low-end phones, which are used by people who need only the basics: calls and SMS, without bothering about Wi-Fi, internet, apps, etc. etc. .
On the "opposing front", phones with big screens are high-end phones (Galaxy Note, Galaxy S3, HTC One X to name a few), bought by people willing to spend more money, usually because they know what they need from a smartphone.
This said, it is only a natural consequence that if one has a big screen (---->user with needs from a smartphone) will browse more on the internet, will use his Wi-Fi to check emails, social networks.
2 years ago on Study Shows Customers Like Bigger Screens