PageSize is the target page format, which dimensions given in physical units (mm, in, pt etc). htmlWidth can bee seen as a width of a virtual “web browser” frame. htmlWidth is mapped to pageSize.width (minus horizontal margins).
Thank you for the link, it made things more clear for me.
So, as I understand, there is no any strict recommendation that HtmlWidth should be less than PageSize.width, it just impacts the scale how PDF is generated (the more HtmlWidth the smaller scale). Correct?