It's probably a formatting problem with the specific book (or something about it that the russified firmware doesn't play nice with is possible too). I downloaded the
free version from Kindle and had Calibre convert it for use on my Sony 350 and I have no problem with the dictionary and different font sizes. It was just a mass import of books to the Sony, so I used no special settings at all.
If you haven't downloaded from Amazon before, you don't need a Kindle to do it, just install their free version for the PC and you can download anything with it. There's obviously no DRM on the book since Calibre didn't have any problems converting it to epub.
Edit: You might also try converting the book with Calibre from epub to epub and see if that alone doesn't correct the problem.