Well Microsoft like putting lots of information in their latest formats that's not really needed but it helps other Microsoft applications process the same data. I also think Word includes images at full size (maybe even uncompressed) despite what size you have formatted them to in the document.
Of course the basic word formatting information is very substantial if you've done anything other than plain text. Try saving an empty document and see what size that is. You can see why people use PDF so much on the web. It is a lot worse if you have 'allow fast saves' enabled as this adds several versions of the same information in the file, also watch out for 'Track changes'. So you may get significant savings by altering your options.