I'm assuming this is due to the lower average signal level of symphonic recordings? Rock music is mastered closer to 0db due to the well-publicized loudness war.

Obviously, this doesn't affect bandwidth in uncompressed audio, where every sample contains every bit, so filesize = bit-depth * sample-rate(hz) * T(seconds). But in compressed audio, average signal level can make a significant difference in filesize/bandwidth.

(Who would have thought a network would be a casualty of the loudness war... harhar)