Ive recently started tinkering with Google Cloud SQL - PostgreSQL.
I have created an empty database and over 4-5 days its storage usage has grown to over 20GB. Its just keeps going up, but there is no data in the database. Its not even being used.
Does anyone know what would be doing this and how to stop it?
Yes, this is most likely due to Point-in-time recovery which will show an increase to your storage every few minutes. You are able to keep automated backups enabled while disabling point-in-time recovery. Once you disable it the binary logs will be deleted and you will notice an immediate reduction in storage usage. That said, according to the documentation: "The binary logs are automatically deleted with their associated automatic backup, which generally happens after about 7 days."
To disable point-in-time recovery:
Most likely you have turned on the automated backups setting. You can confirm this by clicking the backups tab in your Cloud SQL instance. Be careful with disabling and deleting backups in case you will start using your database later!