This is Schoenberg's chamber size reduction of Mahler's piece, and if that intrigues you, than you'll enjoy this recording.  A small set of strings, woodwinds, piano, this and more is the 'orchestra.'  The effect is one of obvious intimacy.  Crescendos and long, sustained notes are obviously foreshortened in this version, and diminuendos become even quieter.  The singers can produce a more natural sound since they do not have to force their way through the thicket of orchestral sound [since Mahler never heard this work performed, he never had a chance to adjust any of the balances], and the score itself provides the analytical approach to the structure and texture of the composition that a conducter would normally have to provide as a choice, thereby putting less emphasis on other aspects of the work.  Herreweghe leads some very musical playing with a nice sense of intensity just contained, and it is wonderful in a way to hear singers able to perform this piece at whisper level.  That built-in exaggeration means that the quieter movements gain greater benefit in this version that the louder ones, but that is to be expected.  The recording is transparent and beautiful.