I just got a new Iphone 5S and also noticed my usage has gone up considerably.
I was just checking my data log for my new cycle which is only two days old and have seen significant MB of data usage in the early morning hours while I'm asleep and I'm on WIFI in my house. The phone is next to me on the night stand.
How could this be? I've read that sometime people leave apps running in the background that might account for that but I'm on WIFI in my house so it shouldn't be using any data at all. I've also noticed smaller amounts of data usage registering during times of the day when I'm in my house on WIFI.