I have a habit of finding out which application is downloading data from the internet even when I am not accessing the internet through a web browser, email client or chat platform etc. I use TCPView application in Windows whenever I see the WiFi LED blink in my router and it shows complete details of which programs are accessing which domain and download how much data. But in Android there is no such utility available that can display live stats for the internet access and data transfer for different apps in the same fashion as the TCPView does. But you can use a free app called Network Monitor to see which apps are accessing the internet and how much data they have downloaded or uploaded so far.
The Network Monitor has three sections Apps, Sockets and Interfaces. The first section Apps shows you all the running apps that have or are still downloading/uploading the data from the internet. For each app, you can see the UID, app name and the amount of data downloaded/uploaded. The data downloaded is written below the amount of the data uploaded. For example, in the following screenshot, YouNow app has downloaded 323 MB of data.
In the Sockets section, you can see all the open connections in use on your Android device. These connections are not yet closed by Android, though they could have been closed by the remote server. For each connection, it shows the local and remote IP address, protocol type, and the status.
You can also see all the interfaces supported by your Android device in the Interfaces section along with the statistics for each of them. The stats show how many bytes or packets have been received or dropped by these interfaces.
Verdict: If you are wondering why your Android smartphone is downloading so much data, then perhaps some rogue app is busy downloading something in the background. You can pin-point such apps using the free Network Monitor app for Android. It shows the amount of data transferred by each running app, the list of open connections and a summary for various interfaces present in your Android smartphone.
You can get Network Monitor for Android from https://play.google.com/store/apps/details?id=com.keustdev.tcc.app.