Discrepancy in AWS RDS Storage Usage

Hello,

I'm currently using AWS RDS for PostgreSQL and I've encountered an issue with storage usage. My RDS instance is allocated 20 GiB of storage. However, when I check the size of my database using the pg_size_pretty(pg_database_size('my_database')) command, it reports that my database is only using around 2279 MB, which is significantly less than the total allocated storage.

I understand that the total storage usage includes not only the actual data but also system files, indexes, logs, and backups. However, it seems unusual that these elements would consume over 86% of the total storage. I have not recently deleted a large amount of data, performed a major version upgrade, or run a large batch job or heavy write operation.

Could you please help me understand why there is such a large discrepancy in storage usage? Is there a way to get a more detailed breakdown of storage usage in AWS RDS?

Thank you for your help.