It's encoded with a variable bit rate and a target of 96Kbps. Compression level of 10, with 10 being the highest quality and slowest to encode. These are the default encoding settings for libopus with ffmpeg ( which is what the audio encoder uses internally ). There are various tools for checking the quality of an audio file, such as ffmpeg, which you could use to find out more about a particular file.
Opus is generally considered one of the higher quality audio codecs. You can see a table comparing relative performance of Opus at different bitrates on this page.
We certainly haven't had any complaints about the quality of the audio encoding or the size of audio files that I'm aware of. If we wanted to provide users with the option of reduced size or increased quality we probably could. Although we would have to modify how the audio encoder dialog worked, and likely make it more complicated. Create a feature request if this is something you'd like to see.