We've just released automatic disk shrinking for Postgres databases using network-attached storage. When your database is using less than 12.5% of the allocated disk space for more than 72 hours, we will automatically shrink the disk to the next available size. This helps you save money by only paying for the storage you need.
This can be disabled on a per-database basis from the "Storage" tab on your database's Clusters page by either selecting "Automatically scale disk size up only" or disabling autoscaling entirely.
Read more about autoscaling.