MongoDB will erroneously allocate new space at essentially a constant rate for the following:

• An insert/delete only workload
• usePowerOf2Sizes used
• essentially constant, at least bounded data size
• Documents greater than 2MB.

A common use case where this applies is where a collection represents, for example, the last 48 hours of some data. Over the 48 hours data is constantly inserted, at some point old data is deleted in bulk.

This results in a constant data size resulting in a linearly increasing database on disk.