I have read every "help" source I can find so far and not found an answer to my question. I have created a word document which is a template for a book - it is not saved as a .dotx file, just a .docx. It contains some introductory text and illustrations, and a lot of empty space in a formatted heading layout for information to be pasted and photos to be inserted. The "empty" document is 1.2 MB, of which the illustrations are 1.1 MB. The full collection of 33 photos (jpeg files) to be inserted is 2.8 MB - average photo size is 85 kB. Once the text is pasted in to fill the empy space, the file is 1.4 MB (this includes the original 1.2 MB of the template file). When the 2.8 MB of photos are added, the file is 39.4 MB. How can that be, when the individual elements of the file total about 3.2 MB? All the photos have been compressed. What else can I do to reduce the size of the file?
Opening the image in Paint or something similar then using Ctrl-C (copy) and Ctrl-V (paste) is one thing that will cause this kind of horrifying bloatage.
When inserting images you should almost always use the Insert->Picture menu option as this will insert the image in (nearly) the exact same format as your source image. The same is not true of Copy-Pasting the image as Word does not know what format the image data on the clipboard is in (the clipboard will use a raw image format) and generally converts it to PNG that will keep all the image data that was present on the clipboard.
PNG is nowhere near as good at compression as JPG for photographic images and a typical JPG to PNG conversion like the Copy-Paste image insertion method will generally result in the file size balooning more and more as the source image size gets larger.
I have seen the picture insertion tool shrinking JPG files size, presumably it defaults to a JPEG compression setting of 85 and applies it on insertion of the image, but I have never seen it unduly making the images larger.
I just tested and it reduced a 600kb image to 120kb (which corresponded to JPEG compression 85 on the original) but that same image when saved to 50kb (JPEG compression 50) stayed at 50kb when inserted into Word
In almost all cases you want to use Insert->Picture even if it means that slight annoyance of saving the image first then hunting it down in the Word file select dialog.
Click on Picture, then you can see an extra tab named as