A colleague just sent me an .rtf weighing in at 9Mb for three A4 pages and three screenshots.
Just out of curiosity, I removed the three graphics – size of .rtf without graphics, 34kb
I saved the three graphics as external .png files – total size, 114kb
To complete the experiment, I then put the graphics back into the document with insert + link …

34kb (text) + 114kb (graphics) = 4,289kb (rtf)

Now, I realise (and have checked) that .docx does a better job; but this document was an extract from another system that only delivers .rtf – and I'm mildly curious to know what's behind the inflation.