Allocated storage RDS instance

There are several factors that could be consuming the additional space in your SQL Server RDS instance beyond just the size of your databases. Here are some potential reasons:

1. Transaction logs: SQL Server keeps transaction logs which can consume significant space, especially if they are not being regularly truncated or backed up.

2. Temporary files: SQL Server creates temporary tables and files during transactions, which occupy space on the instance.

3. Write-ahead logs (WAL): These logs are used to ensure data integrity and can take up considerable space.

4. System databases: SQL Server has system databases like master, model, msdb, and tempdb that consume space in addition to your user databases.

5. Backups: If you have automated backups enabled, these can consume space on the instance before being moved to S3.

6. Database snapshots: If you're using database snapshots, these can occupy space on the instance.

7. Unused space within database files: Even if your databases are only using 500 GiB of data, the actual database files might be larger due to pre-allocation or fragmentation.

8. OS and RDS system files: The operating system and RDS management components also require some space.

To investigate further, you could:

1. Check the size of your transaction logs
2. Look for any large temporary files
3. Review your backup and snapshot settings
4. Check for any orphaned files or bloat within your databases

If you're consistently running low on space, you might want to consider enabling storage autoscaling (if not already enabled) or increasing your allocated storage. Keep in mind that you cannot decrease allocated storage for an RDS instance once it's increased.
Sources
Modifying the storage for an RDS Custom for SQL Server DB instance - Amazon Relational Database Service
RDS Using More Space Than DB Size | AWS re:Post
PROBLEM WITH STORAGE OF MY POSTGRES RDS INSTANCE | AWS re:Post