I used to use about 3 GB a month. When I got a the Droid Turbo my data usage went up to 6 GB a month doing nothing different. I found this odd that the phone systems take so much more data to keep things updated.
Youtube somewhere along the line started streaming in 1080p, which is 2.25X as much data as the same video in 720p (I think, and not sure how compression factors in).
The other thing I've noticed is more frequent app updates, and larger apps.