My wife has several Writer documents with text and images (.jpg photos) that used to be reasonable in size. After having had them edited on another computer at a workshop - I wasn’t there so don’t know exactly how - they are still in .odt format but two things happened: All styles were gone and the documents got about 10x larger! I found the reason for the large size to be the photos.
To address this problem,
a) how do I see properties for images in Writer (did they become bitmaps)?
b) is there any “minimizer” function to make the photos smaller?
Documents in OpenDocument file format are stored as compressed zip archives (from LibreOffice Help). Embedded photos are stored in a Pictures directory in the zip archive. You can open the .odt file with an archive manager (you may need to rename the file.odt to file.zip to do this) and view the file sizes.
Resizing: I made a test document with a large embedded JPG image. I extracted the image from the .zip file, resized and reduced the JPG quality, replaced it in the .zip file and renamed it as .odt. The document size went from 1.4 MB to 0.11 MB.
Thankyou for response. Wow, had no idea an .odt file was an archive! I named it .zip and opened it with ZipGenius. It turned out my used-to-be 300kB photos now are 2MB OLE objects - what now? Can I make them images again?
When I inserted a JPG as an OLE object I only got part of the image (bug?). The file size was inflated compared to inserting a JPG directly. I was able to right-click on the inserted OLE image and export it as JPG (I only got what was displayed). You might try that (on a copy of course).
Thankyou for your input! I suppose we could say that the answer to the question “Is there a way to compress images in Writer” is “No, not for normal users” 
What I will do is use “Compare document” to get new text in old documents which solves most of my problems for the time being.