The storage space allocated to your Amazon DocumentDB (with MongoDB compatibility) cluster will now dynamically decrease when you delete data from the cluster. Amazon DocumentDB is a database service that is purpose-built for JSON data management at scale, fully managed and integrated with AWS, and enterprise-ready with high durability. Previously, when Amazon DocumentDB data was removed, such as by dropping a collection, the overall allocated space remained the same. The free space was reused automatically when data volume increased in the future.
Source:: Amazon AWS