In Docs: How do I reclaim disk space? when I drop a collection from a database

You do not need to reclaim disk space for MongoDB to reuse freed space.

Further in the Empty Records section section:

MongoDB maintains lists of empty records in data files as it deletes documents and collections. MongoDB can reuse this space, but will not, by default, return this space to the operating system.

Improvement Required:

We need to explicitly mention that:

"The space will be reused by other collections in the same database only and not by the collections in the other databases as each database in MMAPv1 has a separate file."