PHP XML 10MB Limit

php_libxml_10mb_limit.txt

Other people have run into this issue:
Warning: DOMDocumentFragment::appendXML(): Entity: line 1: parser error : CData section too big found in /magento/lib/internal/Magento/Framework/View/TemplateEngine/Xhtml/Template.php on line 60
https://github.com/magento/magento2/issues/4695
https://github.com/magento/magento2/issues/8084

Not saying they figured much out, but other people have been experiencing the issue as well.

The limit comes from the libxml library that PHP uses for XML processing:
http://php.net/manual/en/domdocument.loadxml.php#113676

libxml introduced a limit for safety purposes, with the ability to pass an option to override the default 10MB limit.
http://xmlsoft.org/html/libxml-parserInternals.html#XML_MAX_TEXT_LENGTH

That was introduced in libxml 2.9.0 release in Setpember 2012.

You can check to see what version of libxml is on a system:
php -i | grep libxml

The appendXML method is not part of the DOM standard, and the DOMDocumentFragment::appendXML method does not provide the option to pass LIBXML_PARSEHUGE to the method like you can with DOMDocument::loadXML.
http://php.net/manual/en/domdocumentfragment.appendxml.php

It looks like it's the parser that complains about the size, so it may be possible to work around the issue and instead of using appendXML, use loadXML and implement appendXML differently to include the options.