How does Opus differ from MP3 when comparing both in relation to the file size and quality. For example if I were to convert a 320kbps or 128kbps file that is 10MB or 4MB in mp3 format to opus, what will I get?
Opus is a much newer, state of the art technology than MP3. Any time you encode audio into MP3 or Opus, you never get a perfect, exact copy of the original file. There is always some distortion added and some things missing. This is by their design. They are not meant to perfectly preserve all of the information, they are meant to make small files. To answer your question, you would get a file that has all of the distortion as the MP3 did, plus whatever distortion Opus encoding added on top of that. This article is very relevant. You're getting further and further from the original each time.