http://www.sphinx-doc.org/en/master/theming.html#builtin-themes

> body_max_width (int or str): Maximal width of the document body. This can be an int, which is interpreted as pixels or a valid CSS dimension string such as ‘70em’ or ‘50%’. Use ‘none’ if you don’t want a width limit. Defaults may depend on the theme (often 800px).

Maybe, all you we need to do is adding "-D html_theme_options.body_max_width=none" option to sphinx-build when building html for htmlhelp, in Doc/make.bat.

I confirmed this option works with "make html", but I don't have environment for building htmlhelp for now.