After investigating this issue, we have found that old circuit files are kept on some players' devices rather than getting replaced by new ones, causing the game size to increase.
This issue has been fixed with the release of Update 15: Re-downloading a track will automatically delete old files stored on your device for this track. This means that each time we update a track, you will re-download the newest version of the track, which will also delete the previous versions of this track from your device.
Still, please note that we have not updated the tracks all at once with the release of Update 15, therefore, if you would like to delete all of the old track files on your device without having to wait for us to update all of the tracks in future patches and updates, then we invite you to reinstall the game.
The game should take up around 3-4 GB on both Android and iOS devices with all tracks downloaded. If your game takes up more space than that, then please feel free to reinstall the game.