google-cloud-sql-postgresql-storage-keeps-growing, how to solve?

That happens if :

1. Automated backups are enabled 
2. Point-in-time recovery is enabled

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:

• Select your instance
• Select Backups
• Under Settings select Edit
• Uncheck box for point-in-time recovery

To disable Automated backups:

• Select your instance
• Select Backups
• Under Settings select Edit
• Uncheck box for Automated backups