If a document's text is processed when importing, any text that is parsed from the document will be stored in a full-text search index in the .urd file to enable fast searching the text content, which will take up space in the .urd file.
If you don't want this to ever occur when importing, clear out
Tools | Options | Import | File extensions to parse text.
To remove all indexed text from an existing database, see the
Delete all auto-generated keywords sections of
http://www.kinook.com/Forum/showthread.php?t=2825, and then compact the database via
Tools | Compact and Repair.