I just reconverted a few DOCX files with calibre 4.9.1 that were previously converted with calibre 2.x, 3.x, and 4.x. The epub file sizes didn't change by more than a few bytes (probably minor metadata changes).
You can reinstall older calibre releases after uninstalling the current one - you won't lose libraries or settings. If you are using Windows you could install an earlier version of the portable version and leave 4.9.1 in place. See
calibre: Previous releases
Added - if you report the problem at
Bugs : calibre you could attach the docx and mark the report "Private', then Kovid Goyal would be able to take a look at the file etc.
BR