Application data usage location in android

Hi all,
I need to know where each application data usage are monitored in android.
I am planning to hack(ethically) by having tie up with that application team.
For that I need this information,
I am able to see UI of each app usage in settings
Watsapp - 2mb
Hike - 5 mb etc
But I need the file where this data are stored.
And is there way to edit that file in rooted mobile.
I don't know whether this is the right place to ask this question but I am sure there are lot of
Brainees are there here.
Thanks in advance.