Ok, fair enough. I suggest you put a warning about that in the doc, as it's not immediately obvious that it will be half of what you define. Furthermore, if I put the font-size explicitly when adding something, like so:
$docx->addHeading('My heading', 1, array(
'fontSize' => 26,
));
$docx->addText($text, array('pStyle' => 'myStyle', 'fontSize' => 32));
It doesn't divide it by 2; the font size is 26 for the heading, and 32 for the text, and not 13 and 16. So it's not very intuitive.