A docx document of Microsoft Word is actual a zip archive. You can open it and see why some documents are so large yet contain only a few pages.
First drill into the folders to find the file that is responsible. In my case there was an uncompressed (not PNG) image in the media folder.
By simply removing the media item I left the document corrupted. Word tried it’s best to fix it and showed me what was recovered. Then I found out the image was used in a bullet defined in the template. But I don’t use bullets.
Word renamed the other images in the media folder to start the numbering from 1. My document of 3 pages has shrunken from 2 Mb to only 140 Kb. I achieved this by removing what wasn’t needed.