"leaness of file size"? RTF? Are you joking?
RTF files are enormous compared with virtually any other format, especially if they have pictures in them.
Eg, to take an example from my eBook library completely at random: David Weber's "On Basilisk Station":
LRF file: 608kb
MobiPocket file: 676kb
RTF file: 1358kb
The RTF version is double the size of the others. For files with a lot of pictures in them, RTF files can be 10x the size of other formats - 20x the size, even.
RTF is a good choice for a long-term storage format, but please convert it to a proper eBook format for reading. The Sony's support for RTF is very poor - eg, it doesn't support pictures or a table of contents.
Using a tool like "Book Designer", it's the work of a few minutes to convert RTF to LRF and that way be able to view the pictures on the Sony, have a proper table of contents, etc.