This is about compression. Not the sort of compression that makes file sizes smaller, though the two types are often confused. Audio compression reduces dynamic range (the difference between loud and soft) in order to make recordings sound ‘punchier’ or – at least perceptually – louder.
One of the advantages of a louder sounding recording is to make albums more ‘impressive’. This is often considered particularly important on smaller speakers, through radio, or via mp3 where most users are not listening in ideal conditions (ie: on little headphones, on the bus).
But audiophiles – and, increasingly, just people who like music – are complaining that too much compression and not enough dynamic range is killing the quality of recorded music. And they kind of have a point.