Why does it use so much space to store in .doc format?

Well, I prepared a document containing two screenshots copied from the Windows clipboard, and, saved it to Word Document format .doc, and, the file was nearly 12Mb. I copied and pasted the same screenshots and text to another document to Word 2007 and the document, saved in docx format uses 200k of size.

Quite a difference isn’t it?

Maybe the file type used by each application to paste the picture is different?

I think bmp in Writer for a pasted screenshot, but I don’t know in Word.

The substantial difference in file size depends not only on the format of the embedded images, but also on the structure and compression of the two formats. The old .doc format is very complex and verbose; I’m not sure, but I think it does not even compress the text. The newer .docx format is a compressed archive with the text, styles, configuration and embedded images kept in seperate files inside the archive. The old .doc format was never efficient and nowadays it should just be avoided whenever possible. The .docx format is, in the opinion of many developers and standards advocates, also very bad, but if you must use it for exchange or compatibility, it’s always better than the old one.

At the end of the day, OpenDocument (.odt) is the better format ;-).