I have been downloading podcasts and audiobooks from the Internet. Some sites put them in zip files, often including the _MACOSX files. The files are in mp3 and m4b formats and if I drag and drop them from the zip file they are the same size as if I did an extract. I gather they were created using a MAC computer. Were the mp3 and m4b files actually compressed in the archive? If I just drag and drop (easier for what I'm doing) am I missing something they would have picked up from the _MACOSX file?
Yes, dragging & dropping / extracting are synonymous.
Generally speaking, media type files don't compress much.
Also, the _MACOSX file is just for apple's indexing, it allows Apple computers (the one it was packaged on) to search the archive's contents quickly (without having to unarchive it). At least from what I can tell.