I took a look at the odt files you provided. To see it yourself, rename these to .zip files and unpack.
- Writer ODT contains 8.1 KB preview image of document. Word ODT does not. Please note that this is a feature (this image is what you might see instead of Writer icon in file manager).
This seems to leave with only 6.5 KB overhead. When I deleted Thumbnails directory from the zip file, I got:
13.9 KB Writer zip vs 12.3 KB Word zip, which is 1.6 KB difference.
(a fun note, with completely white first page the same document size was 14.9 KB)
- other two notable file size differences are
styles.xml 14.4 vs 8.1 KB
settings.xml 9.9 vs 1.4 KB
styles.xml has some extra styles for graphics, tables, even if these are not used in the document. The largest chunk is outline settings (how 10 levels of headings are positioned and numbered).
settings.xml contains stuff like printing settings, where your cursor was when you saved and stuff. Word’s settings file is empty, it contains nothing useful at all.
- the content.xml file in Writer ODT is actually smaller, 29.5 KB vs 53.5 KB. This means that with longer documents Writer ODT will actually be smaller than Word ODT files. Although these need to be pretty long ones (especially when you need to overcome the thumbnail size). In other words, Writer ODT’s base size is larger, and it is so mostly due to included Thumbnail image.