Unfortunately, I no longer have Microsoft Word, and cannot troubleshoot it. I have seen that happen with other fonts in Word in the past, but never had the opportunity to track it down.
There are no OpenType features that would change the font sizes (at least not in OpenDyslexic), and I can’t think of any reason the font itself would do this.
I’m glad Bookwright shows it correctly, and I’m glad you were able to find a work-around. I wish I could have a better solution outside of recommending a different word processor like Pages, LibreOffice, etc.