Mine shows 702mb of user data after about 6 months of call history. However I don’t find this very unreasonable for the feature set provided.
The app continues to function offline so I could imagine a cache of the information gets generated once the application is launched and permissions granted.
First off, it appears to source contact information, this appears to be standardized because access is backed by the Contacts permission scope. I imagine it caches this information, because it also builds an index of your contacts in order to drive the t9 dialer search.
The phone also offers integration for voicemail and visual voicemail transcriptions. These would either need to be stored in the app, or associated with the data if it rests outside of the apps directory.
Finally there is the call history. It looks like Android has a standard location for this because it has its own permisson scope.
This means, in order to maintain functionality when offline, the app would have to store associations to contacts, the call history, voice transcription text, and voicemail audio.
The look up of this information could be slow to do each time a tab is opened so it likely stores these associations in a local database for quicker access. That local database would need to be stored in the apps directory contributing to its size.