Not a problem on my account, but my partner is having this issue. She streams over 4g nearly every day going to and from work (about a 40 min trip each way). Sometimes at the gym as well. For as long as we can remember she hasn't used more than 120mb a day for spotify, however for the last few weeks has been using anything upto 800mb a day following that same routine.
I started by checking all settings, set stream quality to normal, turn off unnecessary setting like lyrics and broadcasting to Facebook. Still no change.
Second, i uninstalled the app, cleared the phones cache and all temporary files. Before reinstalling i manually check for and deleted any and all spotify related files. Re-install, no change.
Third, I've done a full reset on the phone. Re-installed minimal apps after reset (emails for work, facebook & messenger, snapchat and spotify) exactly the same usage. Have checked after every attempt to fix the issue that stream setting is set to normal. Also checked every time that downloading over 3g/4g is disabled.
For the last 3 days now ive asked her to download playlists at home and only play saved music, and there hasn't been a single MB used by spotify so i can confidently say the issue is only streaming related.
I'm lost now, i stream just as much if not more each day on my phone and my usage is fine. Really needing some help.