org.apache.hadoop.fs.FileSystem.create(File f, boolean overwrite, int bufferSize)
ignores the input parameter bufferSize.
It passes further down the internal configuration, which includes the buffer size, but not the parameter value.
This works fine within the file system, since everything that calls create extracts buffer size from the same config.
MapReduce although is probably affected by that, see
org.apache.hadoop.io.SequenceFile.Sorter.MergeQueue.MergeQueue(int size, String outName, boolean done)
The attached patch would fix it.