My internet broadband package at home is limited to a volume of 40GB. Ever since I bought my iphone 6s, 40GB finishes within half the month. I have disable all auto downloads, auto update, iCloud backup over wifi; but yet my iPhone wastes about 3,5 gigabytes per day for nothong. I can't understand why. Longer I keep the phone connected to my home wifi connection the more my wifi internet volume drops. It doesn't happen over cellular data anyway, only when my phone is connected to home wifi. Please help me to solve this problem.
This article should help you greatly: http://www.wired.com/2015/10/a-new-ios-9-feature-is-eating-your-cellular-data/
There is also a list in settings that lists how much data various apps use. It's most likely Mail or Messages using so much data for push notifications. If you switch to a 5-minutely update rather than push, then you might have better results.