Microsoft Word (like many programs) has two different save options. These options are Save and Save As. The second of these options works as you would expect with your selecting the filename to save the current document in and then having the program save the document in that file.
The Save option (on the other hand) may not be functioning as you expect because the default action is not to save a completely new copy of the file over the top of the existing file. Word actually keeps track of the changes that you make to the file and when the save option is selected the default action is to save information about the changes on the end of the existing file rather than saving the entire file again. This makes for faster saves but means that each time you save a file the file will be made bigger.
You can change this default behaviour by selecting Options from the Tool menu to bring up the Options dialog box then selecting the Save tab. There are a number of options presented on this page that you can change that will affect the way that the Save and Save As options function including one called Allow Fast Saves which by default is selected and which is the reason why the Save function saves changes on the end of the file instead of saving the entire document again.
If you deselect this field and select the OK button at the bottom of the dialog then the Save option will in future overwrite the existing file with a complete copy of the updated document. This will make the save slower but will also make the resultant file smaller.
I suggest that you don't need to make this change. There is an alternative way to work that achieves the benefits of the faster save as well as keeping the file size as small as possible. To do this you use the Save option every so often when you are working on a document so as to save the changes that you have made on the end of the existing file. When you reach the point where the document is complete or you wish to distribute it, you will wish to make the file as small as possible. At this point you use the Save As option instead of Save. You can leave the filename and location as is already filled out and straight away select the OK button. You will then get a dialog box telling you that the file already exists. Confirm that it is okay to overwrite the existing copy of the file and a complete copy of the document will be saved over the top of your previous version. As this is a full save the resultant file size should be smaller than the previous size of the file.
One final note: if you intend to make a Word document available for download from the internet you should first minimize the document size as described above and then use a compression tool to convert the resultant document into an even smaller zip archive. This will then be the file that you upload to the internet. Along with the file for download you may also want to provide links to compression tools so that the person downloading the file can convert the file back into a Word document (most people should already have this but it doesn't hurt to provide a link to a site where anyone who doesn't can get it from). You should also consider providing a link to the Microsoft download page where people without a copy of Word can download the free Word Viewer.
This article written by Stephen Chapman, Felgall Pty Ltd.