Since you don't want to download anything, I hope you have LAME installed already. Here's how you can decode and encode your MP3 file a lot easier than with iTunes.
lame --decode soundfile1.mp3 soundfile1.wav
rm soundfile1.mp3
lame soundfile1.wav
Example session:
% ls -l 01.\ Drive\ My\ Car.mp3
-rwxrwxr-x 1 janus plugdev 6064941 jan 26 2012 01. Drive My Car.mp3
% lame --decode 01.\ Drive\ My\ Car.mp3
input: 01. Drive My Car.mp3 (44.1 kHz, 2 channels, MPEG-1 Layer III)
output: 01. Drive My Car.wav (16 bit, Microsoft WAVE)
skipping initial 1105 samples (encoder+decoder delay)
skipping final 743 samples (encoder padding-decoder delay)
Frame# 5754/5754 168 kbps MS
% rm 01.\ Drive\ My\ Car.mp3
% lame 01.\ Drive\ My\ Car.wav
LAME 3.99.3 32bits (http://lame.sf.net)
CPU features: MMX (ASM used), SSE (ASM used), SSE2
Using polyphase lowpass filter, transition band: 16538 Hz - 17071 Hz
Encoding 01. Drive My Car.wav to 01. Drive My Car.mp3
Encoding as 44.1 kHz j-stereo MPEG-1 Layer III (11x) 128 kbps qval=3
Frame | CPU time/estim | REAL time/estim | play/CPU | ETA
5754/5754 (100%)| 0:09/ 0:09| 0:09/ 0:09| 16.338x| 0:00
-------------------------------------------------------------------------------------------------------------------------------------------
kbps LR MS % long switch short %
128.0 89.6 10.4 91.6 4.7 3.7
Writing LAME Tag...done
ReplayGain: -3.7dB
% ls -l 01.\ Drive\ My\ Car.mp3
-rw-rw-r-- 1 janus plugdev 2405354 okt 21 19:28 01. Drive My Car.mp3
As you can see, the default settings gives you a file with less than half the size of the original, using the original file.
If you don't have LAME, but you do have internet: get homebrew and then brew install lame && brew link lame