Word 2010
saved blank document as .docx 12.1kb
saved blank document as .doc 21.5kb
mail merge A4 21 a sheet labels .docx 23.7kb
mail merge A4 21 a sheet labels .doc 43kb
there must be some other formatting going on, whatever it is I find both those reported file sizes very odd, I have never seen a Word doc of that little size and the 301 kb sounds rather large to say the least for just a sheet of plain labels.
Perhaps your computer is incorrectly reporting the file sizes