5. Wi-Fi Assist
One feature of the iPhones is the Wi-Fi assist. This is basically used when the Wi-Fi signal is very weak and as a result, your cellular data is used to provide some sort of boost to get the Wi-Fi working properly. This consumes a huge amount of data and is very bad if you are trying to cut down the data usage. For this just go to the Settings app and find Cellular. From there, turn of Wi-Fi Assist.