Without getting too much into the internals of storage, MongoDB uses memory-mapped files, which means that the data is stored in files exactly as it would be in memory; it will use low-level OS services to map these pages to memory. The documents are stored in continuous locations in Mongo data files and the problem arises when the document grows and no longer fits in the space. In such scenarios, Mongo rewrites the document towards the end of the collection with the updated data and clears up the space where it was originally placed (note that this space is not released to the OS as free space).

This is not a big problem for applications, which don't expect the documents to grow in size; however, this is a big performance hit for those who foresee this growth in the document size over a period of time and potentially, a lot of such document movements. The paddingFactor field, that we saw in the Viewing collection stats recipe, gets updated over a period of time...