The app usage data presented in Settings--> General --> iPhone Storage --> {app name} does not include usage data from log files, cache and other system data that is written to disk. For example: an app showing a total of 1.5GB consumption between "App Size" + "Documents and Data" may actually be consuming much more in the "System Data" bucket.
Solution: To clear the applications writing data (logs, cache, etc) to "System Data", you will need to remove the app completely by Remove App --> Delete App & Contents --> then reinstall the application from the App Store. I was able to reclaim ~15GB of System Data by deleting Facebook, Twitter and Instagram. Then reinstalling those apps.