This is a problem for me as well!
For example: the Bloomberg app is only 16.7 MB, but it has accumulated 68.9 MB of data. This app data is totally useless as I read the news only once but they get downloaded and stored on my device every single time I check the news...
You can see how much data your apps are accumulating under Settings > Genereal > Usage.
I assume that one way to delete the unnecessary data is to delete the app and reinstall.
Does somebody know an other way? Please help!