Here's a quick and easy guide on how to check which apps use the most mobile data on your device, thanks to the built-in features on iOS, Android and Windows Phone systems.
For iPhones running on iOS 8 and higher:
Go to Settings > Mobile Data and scroll down to see yourdata usage. You can view a list of apps with the mobile data they’ve used. Check out System Services to understand how much data is used by the respective services supported by your device.
For Android phones:
Go to Settings > Data Usage to see a list of apps arranged in the order of which app uses the most data.
For Windows Phone:
Go to the Data Sense app, and swipe right to go to Usage. Apps listed are arranged to show the ones that use the most data.