what's extsort.* (8 - 11 MB) in MongoDB dbpath? Directory _tmp
These files are used when the sort you are attempting in an aggregation exceeds the maximum allowed memory and you have allowDiskUse : true specified.

The relevant pieces of the code are here (where the SortedFileWriter is called) and here (where the extsorter.# files are written to temp by SortedFileWriter). Basically, this is the number of files required by MongoDB to perform the sorting that is being specified in your aggregations.