Rick recently upgraded to the iPhone 6. He set it up just like the iPhone 4s he used to have and his data usage has skyrockted, even though he's using it exactly the same. Leo says that there was an issue where handing off from LTE to Wi-Fi occured, but Apple fixed that. It's more likely there are apps that are causing it, like Facebook which has autoplay on movies set by default. That can easily cause it. Also, with faster speeds, apps like YouTube will automatically bump up the quality, and use more data.
Leo advises going into the settings and disable cellular data for all the apps that he doesn't need it for. There's also a feature in iOS that will show him what apps and services are using the most data.