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
It all depends on which format you saved it. The plain text format is smallest in size but loses all the formatting.
Using Word. have just saved an A4 page of text as a Word Document 19.8KB
The same document saved in Word as a rich text document is 163KB
The same document saved in Word as a plain text document is 4.95KB