Compressed formats does not affect file size of songs. Its the
bitrate. Just use iTunes --> Preferences --> Importing -->
MP3 Encoder --> (select a bitrate suitable to your file
size)
Then right click on the song you wanna reduce in the library and
convert to mp3.
Eg. If you have a 160kbps mp3 song, and you wanna convert to a
smaller file size aka lower bitrate, select 128kbps as your
encoding settings and convert it. A 160kbps MP3 file will be
converted to 128kbps MP3 file.
Converting between song formats reduce the quality
