I've noticed an iPhone 5S consuming quite a lot of data (30-50Mb per day) - it's mostly listed under "Time & Location". I have apps running that use the location a lot (e.g. Strava, location sharing etc), but I don't understand why getting your location uses any significant amount of data at all.
I understand that A-GPS (assisted GPS) uses the mobile network to transfer some information, but from everything I can see it should be a few KB rather than 10's MB. As a comparison I tried an iPhone 5S and Galaxy S7 running the same specific location sharing apps and the usage over the same period on the android was only a few KB compared with the iPhone's 10's MB.
Please note before marking as duplicate: there are many similar questions here asking "how can I reduce this data" and the answer is always "turn off location sharing etc". I obviously do want to keep using the various location apps I'm using so turning off location isn't an option, I just don't understand what it causing ios to use so much data when using GPS (which is largely separate to the mobile network).