It happens at idle, but at a much slower rate than when it is syncing files/folders. I ended up just restarting the app every day, as by the end of the day I would hit 10+ GB usage. It’s a very badly coded piece of software. Since this has been an issue for years as can be seen throughout the forums, I suspect their main mac coder has resigned some time ago and the app has been left to languish, with some minor changes made but nothing significant to try fix this problem.