I don’t know all the details but Ogg is dead, and Opus has all the advantages from low quality (Speex) to high quality (better than Ogg). It’s made by the same guys anyway. And starting at 128 kbps approximatively, it’s “near perfect” quality which means your ears won’t detect the difference with FLAC. So: as small as MP3, as good as FLAC. I love that stuff.