ipod user - wma to mp3 conversion

hi,

i purchased an ipod, and then had to switch my music library to itunes
it did only convert automatically part of my library (wma) into mp3.
so i looked for a file converter and started to convert the rest of my library manually.

it is very slow however, and the size of the files is about unchanged, while i expected the files would be smaller
i would very much appreciate advice on:
- how it is that not all of the files were automatically converted when switching to itunes
- what is the best converter to do the conversion if i have to do it manually
- if there is a way to get smaller files (at the same bitrate)

thanks a lot
adrien