DOC files are (supposed to be) a "legacy" format. Sometimes DOC files were in a binary format and sometimes the DOC file was a plain-text RTF with Word extensions.
DOCX files are plain text XML and other data (e.g. images) stored in a folder tree compressed and stored in zip format. You can change the extension to ZIP and then open them. (Handy for extracting images).
Word 2010 on Windows will just change the extension of the the file to DOC if you use "save as" but it is still a zipped DOCX file.
If you save it as RTF you will probably be able to reproduce the behavior in your Windows environment.
I did a test with a single page of text from a book chosen at random from Project Gutenberg:
- RTF = 205KB
- DOCX = 25KB
- RTF zipped up using 3rd party software = 31KB
So the my best guess is that the difference in file size is due to the ZIP compression.
Protected view? maybe the file is corrupted, maybe you have a VBA macro embedded and they have a 100% lockdown on macro policy, who knows. I would ask that question separately.