Your phone should have a setting option to clear application cache. Applications will store data on the phone for faster loading times, moreso the more frequently you use an app and especially so if it's using the internet like this one is.
I'm sorry I don't have the specific instructions, as I am an Android user. But do a search for how to clear your phone's cache. It should be a pretty basic straightforward task.
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It's bad coding. Alerted the right people months ago, they had no idea or solution, best I found was to delete and reinstall the app. Right back down to a more normal size.
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Within the app, you can clear the cache. The app shows only about 300 MB in the cache, leaving 2-ish GB of unaccounted-for data.