I just created a Word document of 56 pages that was 29kb. I added three levels of headings, paginated headers that changed with headings and a Table of Contents. That made it 30kb.
1) They formatted using styles.
2) They formatted using styles.
3) They did not include too many pictures.
4) Probably not a lot of section breaks.
Understanding Styles in Microsoft Word
Sections / Headers and Footers in Microsoft Word 2007-2016
Basic Concepts of Microsoft Word - from Shauna Kelly