I do love lrf and change all my books to it. This format circumvents the big gap in the font size. So when you have XS, S and M size the steps aren't as big as with epub. All other formatting that comes with the epub book is maintained when you change it into lrf.
As BBeB/lrf was Sonys first standard book format, I guess this is the reason for its better font size scale.
To have an easy way to try it out just download a book from the
Patrica Clark Library in lrf format.
Congratulations! Enjoy your new reader!