Why is "Android OS" uploading so much data? After switching from my Nexus S to a Nexus 4 I noticed that my phone was transferring huge amounts of background data.  Like 70 MB in 15 minutes.  As my data plan only allows 300 MB per month I was not amused.  Even when connected to WiFi, it often (but not always!) was uploading tons of data.

According to the "Network Usage" the culprit was "Android OS".  So I tried to limit the background data of it only to find that "Android OS" seems to be the only "app" which doesn't offer this option.  Great.

Well okay, that's what firewalls apps are made for. 
I configured AFWall+ but that only cured the symptom not the cause.

Next I redirected the traffic to a laptop with a WiFi card and used the usual tools (tcpdump, wireshark) to analyse the traffic.  Result: many many uploads to some Google servers but SSL encrypted so no chance to see the contents.

I kept investigating and found this very interesting article on Android data usage.
Apparently Google has began expanding its cloud-based backups of the phone settings so that it now also does backups of the data of some apps.  So, I turned off "Back up my settings" in the Privacy settings and whoopiedoo, the sending stopped and the data traffic returned to a normal amount!

There is a post in a Google forum with similar experience: Something within the Android System—maybe Backup—is using up all my data allowance.

I started looking in the Android documentation and found that there is such a thing as "Android Backup Service" which apps can use to backup their application data to the Google cloud: Data Backup

And there is even an CLI utility "bmgr" to interact with the Backup Manager.

But I cannot find any information about how the phone owner can find out which application is backing up which data, or how to influence or stop this.  I'd be happy to use the backup service—it saved me a lot of time when switching to my new phone—but the way it is implemented now it's definitely unusable.

After talking to my former coworker and Android expert Izzy who was clueless too, I decided to post this issue here as—like he said—I will find the best experts here.  Well, let me know if you have any idea about this.  Any hint is welcome!
The list of applications with backups as well as how much space each backup uses is on your Google Account Dashboard under "Android Devices".

Unfortunately, there isn't, I believe, any way to control it other than turning it on or off entirely.