Well this is actually a limitation in QtWebEngine/Chromium. They encode each xhtml into a URL data style url and that after encoding is limited to 2mb.
After that it will simply refuse to load. You are hitting that limitation. Qt is aware of the bug but refuses to fix it as it is upstream (Chromium).
See
https://bugreports.qt.io/browse/QTBUG-53414 for example.
After our next release I will look into working around this using file urls and view->load().