Microsoft always uses vast amounts of memory when either creating programs or data files used in their products.
If a Word document contains any images, it will increase the resulting file size appreciable, especially if the image/s used are high resolution. Word has the ability to automatically downsize images when they are inserted into a document, depending on which method you use to place them. In addition, Word also allows the user to re-size an image by dragging with the mouse, however the original (large) file is still stored in the Word document, and will occupy a large amount of memory even after re-sizing on the page.
The solution is to strip out any unwanted or unnecessary images, including logos etc, in the document, or ideally re-sample all images in image editor before using them in Word.