The high data usage of the iPhone 6 is usually unintended but almost always preventable. Every new generation of the iPhone is reported to use up twice as much data as its previous predecessor. Most apps in your iPhone require some sort of connection to the internet, be it the Wi-Fi or the data. Those apps also need a constant connection to the internet to properly function, something like a chat app. While only one of these apps doesn’t really use up a data that much, a bunch of apps could easily drain your data away.
You can actually turn off the internet dependency of each single apps, although it can be pretty difficult to track where your data usage went if you don’t know where to start. The biggest offender could usually be the phone main system app itself, the one that keeps ‘Siri’ and music player app running, usually you can’t turn them off. Then aim to turn off the internet function of all the background apps that you don’t use anymore. Make sure to close out the apps you had used as well cuz often it will automatically download unwanted updates. Uninstalling them would help a lot too.