Why does Pages, export to *.doc a file that is larger than expected?
There is a bug in Pages. When you export a file in ".doc" or ".docx" format it embeds 489KB PNG files (a single one in a docx file, three in a doc file). These files are intended to be used as the background fill on default shapes, but they are present whether they are used or not.

I documented the bug here.

And reported it to Apple as rdar://17089255.

At the moment the simplest solution is to open and close the file in another application like Microsoft Word or LibreOffice. They both strip out the unused PNG files.