The happy medium between small file sizes and true ... be to encode MP3s using a 'variable bit rate' instead of a 'constant bit rate'. Using VBR allows the MP3 encoder to use lower bit rates ...

To export Audacity files ... choose your MP3 settings. As a general rule, you'll get the best results by choosing "Constant" for the "Bit Rate Mode" rather than one of the variable bitrate options.