Se houver imagens no documento RTF, você poderá verificar o artigo deste KB da MS :
Sintoma:
...If an EMF, a PNG, a GIF, or a JPEG graphic is inserted into a Word document, when the document is saved, two copies of the graphic are saved in the document.
...a Microsoft Word 2000 document that contains a JPEG graphic that is saved as a Word 2000 document may have a file size of 45,568 bytes (44.5KB). However, when you save this file as Word 6.0/95 (.doc) or as Rich Text Format (.rtf), the file size may grow to 1,289,728 bytes (1.22MB).
Resumo da resolução:
To prevent Word from saving two copies of the graphic in the document, and to reduce the file size of the document, add the ExportPictureWithMetafile=0 string value to the Microsoft Windows registry...
Talvez haja alguma ajuda aí?