You may know this, but all MP3 encoders are not created equal.
The LAME encoder that Audacity (and Sound Studio) uses is not as efficient as the one in iTunes.
Case in point: I was just cutting some audio, and a 19-second WAV file of 1.6 Mb compressed to 300 Kb using the LAME encoder in Sound Studio but only 192 Kb using iTunes. I've had similar results with other files using Audacity.
So while it may be an extra step to fire up iTunes and run the file through it, it's worth it since iTunes is a free downoad for Mac or Windows.