Depends on several factors, such as fonts you use, formatting, compatibility settings, etc.
I created a Pages file with a full page of solid text and saved it in several ways. The size differences were amazing.
Standard Pages doc using Helvetica Neue font: 909KB
Standard Pages doc using Arial font: 907KB
Standard Pages doc using Times font: 873KB
Exporting as a .docX file: 9KB
Exporting as a .rtf file: 4.3KB
Exporing as a plain .txt file: 3.9KB
The reason(s) are simple. Pages saves a fully formatted-aware document... even if you haven't formatted it. It saves the page layout metadata, preview images and more.
Here is a screenshot of the Helvetica Neue file I used. Pages documents are actually package files that you can open and view.
Since all I did was paste a bunch of text in my file, I have no idea what this "tile_paper_medgray-9.jpg" file is or where it's being used, but it is responsible for 443KB of that 909KB file size.