Connect your Apple device to your PC and click on Clean Up iPhone, iPad or iPod touch.
Then, hit the Scan Now button and have a look at the results.
AVG Cleaner for iOS cleans up the following leftover files
App Cache and Temporary Files folders which contain data that‘s needed while the app is running. This includes, for example, downloaded map data that you looked at when you were on your last vacation, videos you streamed using a YouTube app, and temporary files that can be created from scratch if the apps really need them.
System Logs that are created and forgotten after an app crashes.
Thumbnail files which are created every time you browse a photo related app. These are used to display the little previews, but the app may forget to delete them when they’re no longer necessary.