I did a little test myself. I had a WAV 35.602 Mb and encoded it to OGG VORBIS and MP3 formats (the same quality).
Quality: bitrate: 160 kbps, sampling rate 44.1 kHz.

Results:
OGG: size: 3,400,606 b. MP3: size: 4,134,139 b.
We got unbelievable results here. 734 kilobytes diference. And I must say that OGG VORBIS sound quality is better than MP3.