I think it’s a bug, but I don’t find anything about it all around the web…
Have you reproduced it?
I know that importing mp3 songs as sound effect (via content pipeline) will increase about 5/10% in size (when converted in wav).
The real issue is that the same mp3, imported as song (via content pipeline) doubled its size (they are converted in wma) !
I am very interested in your code for decoding .ogg at runtime, and I will be very happy to review it 