Some codecs only work at low bitrates and preserve only narrow bands of frequencies. Some codecs work only at mid bitrates and preserve wider bands. Some codes only work at high bitrates and preserve only the widest bands; you can't get then to drop more frequencies for better savings even if you wanted to. Opus works on all bitrates and gradually and dynamically removes frequency bands as the bitrate drops. Vorbis preserves more or less the same frequencies as Opus at the same bitrate, but loses frequencies a bit faster as the bitrate drops. MP3 drops even faster. AAC works very similarly to Opus, but can't output low bitrate streams.
To compare codec efficiency you would need to do subjective comparisons to see how often each codec achieves transparency (when people can no longer tell if the sound has been compressed or not) at a given bitrate with various types of sounds. This has also been measured, and it's agreed that Opus is basically transparent at 128 kbps. MP3 needs twice as many bits to get the same quality, so Opus is twice as efficient.