How to determine which image(s) in a word document are unusually large?

You can open the docx file in a program like winrar and see all the components (including images) that go into making the document.

Images are stored in the word -> media folder

If you save the Word document as a Web Page (*.htm; *.html), embedded images will be extracted to a <document>_files folder in the same location as the new HTML file. From there, you can sort by size and find the culprit.