Former TUAW peeps, now Apple World Today[1]:
These results are not surprising given the larger size of the iPhone 6 Plus screen. The iPhone 6 Plus makes it a breeze to browse websites, watch movies and navigate maps on the go. You are more likely to grab your 6 Plus and look up something right away instead of waiting until you have your iPad or Mac in front of you. It’s also big enough that it becomes an iPad replacement for some people, allowing them to use one device to met all their internet needs instead of two.
Something similar happened when LTE went big-time a few years ago. If you streamline the pipe, flow through the pipe can be increased. Therefore, people will consume more content per given time differential, due to the increased flow capabilities, resulting in more total content delivered. Similarly, if you physically make the diameter of the pipe bigger, people will consume more content as well. iPhone 6 Plus users have both better flow through their pipe via LTE, and they have a bigger pipe due to their larger display.
It seems the carriers understand this development, and have offered revised data plans to help customers cope with their increased data usage.