I use opus's built-in 7z compression and 7zip to compress the same files. The 7zip file is much smaller than the built-in 7z.
Did you use the same settings in both programs?
Opus literally uses the same 7z.dll that 7-Zip does so there won't be any differences unless they are due to compression settings or differences between 7-Zip versions.
I tried more times and I found this problem when compressing a set of xls files, using the default settings. Do you need me to send you these files?
The default settings in Opus and 7-Zip are not the same settings.
This is not a bug, just a configuration difference. You can configure Opus differently if you wish.