Why does the file size change when you change the "open with" option in Get Info?
Not sure but it MIGHT be the MS Word icon that takes the extra space.

Try this, get info on the file after you have changed it back from an MS Word document.

Click on the icon in the upper left corner

Press COMMAND-X (cut)

close the window

re-open it and see what the size is....

Just spitballing here...