When you open an XML-based Word DOCX document in Pages, there is a translation process that converts that Word content to the Pages document architecture. That Pages architecture is not based on XML and does result in a larger result for that reason. That is an Apple "black box" translation tool and you have no control over the resulting Pages document size or even a guarantee that the translation will be successful.
Microsoft released Office 2013 in January of that year. The very last update for Pages '09 was in Dec 2012, so likely ignorant of any changes in Word 2013. No version of Pages after Oct 2013 is based on Pages '09, so when you export a current Pages document to Pages 09, you are losing current Pages features, and some translated Word features not supported in Pages '09. Yes, you will likely receive a smaller exported document owing to a decrease in content and the Pages '09 XML internal document architecture. But is that worth the content and interoperability loss? I never export Pages documents to RTF as that just introduces another unnecessary format. It is based on a proprietary Microsoft binary format that predated the DOC and DOCX formats.
If you were using Microsoft Word, your 13 KB Word document would open without translation in its native format at the same document size. You could edit, or save it without any translation process into another document format.
Frankly, I don't pay attention to Pages document sizes as I configure my Macs with sufficient storage in the first place. Some simple Pages documents containing just text can be opened in recent versions of LibreOffice and then saved as very small Word documents.
