For me space is not an issue, just for example I compared a straight .txt file. Then converted it to an .rtf.
461 KB (472,855 bytes) as a text file.
464 KB (475,447 bytes) as an .rtf file.
For my next test I went to manybooks.net and downloaded Jules Verne's 20,000 leagues.
Epub = 278 kb
RTF = 561 kb.
So roughly twice as big. And if your device has limited storage I can see why you would prefer epub. But I have a 4 gig SDHC card in my reader and its only 1/4 full. So why should I care about space?
What I care about is how smoothly it delivers the words.
I don't need pictures of covers, TOC, etc.
Just feed me a new page full of nicely arranged words every time I press the button.
RTF gives me the nicest looking easiest to read books time after time.
Plus I don't have to add on 3rd party software if I want to open one with my computer. Just click on it and its there.