Flac is an even clearer example.
You can set it to use different compression levels.
All resulting in the same bit-rate output, but with different file sizes.
(but also pretty much negligible in practice)
Well, FLAC is variable bit rate, which is not what we're talking about here, but even on its face this claim is decidedly not true. The definition of compression is changing the bit rate.
It looks like we are slightly talking past each other.
I was interested in differences in resulting storage file sizes.
For lossless flac, that will most certainly depend on the (0-8) setting. I am sure you didn't intend arguing that?
I have always been convinced that different lossy algorithms would perform differently in resulting file sizes depending on how sophisticated or advanced they are.
I did some quick testing with dBpoweramp, which has the option to refine any fixed cbr setting to 10 different quality levels.
I always assumed that higher quality settings would not only (substantially) increase the encoding time, but it also would result in larger file sizes, since more refined audio information would need to be stored.
It also has the option to select e.g. 'stereo' encoding vs. 'joint-stereo' encoding.
Again, I have always assumed and understood that 'stereo' would take up more storage space then encoding 'joint-stereo'.
But trying out these different settings (with the same 320cbr setting) indeed always results in the same file size.
That's really surprising to me. So I'll probably need to re-adjust my understanding on this matter.
But to return on-topic: I was responding to the notion that some users would use Opus also at higher bit-rates, because it would result in lower file-sizes than mp3, with comparable (or even better) sound-quality.
And on that, I have not found any reliable proof or abx testing.