Can you go to System Settings - Connections - Data Usage - Select WIFI and check what apps are eating your data?
There is a new feature, Auto-Backup. It will back up your entire phone gallery with video to g+'s private gallery.
Perhaps that's the cause for large data usage.
There are also location services and location history, which use the Wifi to collect SSIDs broadcast from any wifi networks it comes into contact with.
That's how google know where you are, as each wifi has it's fixed location, and even tho you are not connecting to it, google collect that info and compare against it's database, being able to precice location without use of GPS.
Hence why Google Now insist on you keeping your wifi on, you collect the SSID as you walk, drive etc...
Anyway, these can consume data, if your have location related services turned on.