The document layout looks like a big table with width=100%, whose cells contain smaller content tables with fixed widths.
In the case increasing of htmlWidth (it is an analog of browser frame width) just increases gaps between the content tables and has almost no impact to the document height.
If the nested content tables had width given in percents, at some values of htmlWidth currently multiline content cells could have only a single line of text (which would efficiently reduce the document height and would help to achieve the needed aspect ratio at bigger zoom factors).
You could just play with the document layout in a web browser – just load the doc and resize the browser frame width. With current layout even if you stretch the frame very wide, the document height will reduce insignificantly. For a better visualization I would recommend to temporarily set border=1 for all tables.
If you want, for the test you may use PD4ML in GUI mode:
java -Xmx512m -jar pd4ml.jar