I don't think that's actually true?
E.g. 160kbps only indicates the data transfer rate, and doesn't directly relate to the size of the source file.
Depending on the efficiency and complexity of the de- encoder, and the contents of the audio file, the file size and the data rate will not be (easily) mathematically related.
So your two files just happened to be less than 300 b different across 50 minutes?
No, I don't think they just happen to be near identical in size.
I think it is because of the efficiency of the encoding algorithms of opus and mp3 being similar.
To show my point: some mp3 encoders have an 'hq' setting.
That will not influence the set bit-rate, but it will (theoretically) give a higher sound quality.
And a slightly larger file size...
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)
I am sure more space efficient lossy audio algorithms could be developed (or already exist).
But the minor gains in storage space will probably not weigh up against the more computing power needed. (battery life of portable players)
The upsampling to 48khz that opus does, is motivated by the fact that cheaper dacs have some audio degradation when playing at 44.1khz.
(there is some factual and justifiable math behind that)
And they believe that the degradation from upsampling to 48khz is preferable over the sound deterioration that occurs with these cheaper dacs playing at 44.1khz.
So again, opus seems to be mainly targeted at low to medium audio quality demands.
And for streaming purposes. It is said to be designed to be more robust compared to other lossy codecs.
(better latency behavior)
So it seems to be safe to conclude that there are no good reasons to use opus at higher bitrates on 'good' and modern equipment.