In CSS 2.1 specification (https://www.w3.org/TR/CSS21/fonts.html#propdef-font-size) font size values aren't defined for font size keywords like x-small, small, this has recommendation only about the scale factor based on the medium font size. The recommended scale factor is 1.2 and most used medium font size is 12pt.
User agents (browsers and MS Word) use own sizes for the size keywords.
The sizes used in Aspose.Words are the same with IE and Firefox. MS Word 2013 and Chrome use the different sizes (x-small is 10 pt).
Moreover, different versions of MS Word and IE can use different values.
So, we don't want to change the sizes used in Aspose.Words. We currently see only one way - add an option to specify these values. We have LoadOptions class, we can create derived class HtmlLoadOptions for this purpose.
So, this is not a bug, this is a minor feature request instead and we don't think that this request will be solved quickly because we have more urgent requests.
Changing the source HTML files can be the quickest way for you to solve this issue.