I have had spacing issues but never such a difference in font size in the middle of a letter. Been using Microsoft since Windows 2.0. Ah yes, the days of having to type correspondence quality command to your printer.
The only way to get a font size discrepancy is using “paste-special” where you paste as “text only” and the target processor’s default font makes the change, then you “save as” without checking the document for such errors. It does clearly indicate modifications apart from the original content document.