I think I just solved the problem. It has to do with the settings on the "Normal" paragraph style. There is an "automatic update" setting that changes all text in the document that is designated as that style. Since I never mess with styles, my entire document changes every time a change formatting of any text.

Why it increases the file size like that, I have no idea.

Instructions for changing the setting are found here:

Format Changes Entire Document in Microsoft Word

So far I haven't had the problem recur, but I'm keeping my fingers crossed!