For your audio, you aren't re-compressing your ogg files are you?
If you are really dead set on further compression of your audio (and, frankly, I think you are making a mistake going below ~128kbps), you should at least start with the original, non-compressed files, which should be flac/alac/wav.
Re-compressing lossy formats, be they image or sound, introduces additional artifacts to the output. This is called generational loss: http://en.wikipedia.org/wiki/Generation_loss
Oggdrop defaults to quality 4 (q4). This is a quality mode, so it aims not for a particular bitrate, but a general level of quality. Put a bunch of different content in, though, and the bitrate will average to ~128kbps.
It may sound a bit better than C2's 128kbps output, since it's based on a more recent version of the ogg encoder.
Orchestral music can be very challenging for lossy formats; it tends to have more complex harmonics than typical pop/rock music, and a greater dynamic range. This is probably one reason your files are coming out larger than average.