Index builds limit their memory usage with the parameter maxIndexBuildMemoryUsageMegabytes, which defaults to 500MB. The memory usage calculation depends on the BSONObj accurately reporting its memory usuage in memUsageForSorter().

BSONObj reports its memory usage as the size of the object, but not the capacity of the buffer holding that object. This is problematic because index builds can use significantly more memory than they they should.

We should change the imlementation of memUsageForSorter() to report the size of the underlying SharedBuffer, in a very similar way to  what we did for in SERVER-42931.