export MP3 to be same file size as imported MP3[SOLVED]

Using Audacity on Debian testing. I am wondering why, when I download a MP3 from Amazon, and run it through Audacity to amplify it a few dB, the resulting file size after I export is always smaller…? Is there a loss of some kind? Should I do something differently? Thanks.

To make the exported MP3 the same file size, click “Options…” when you export MP3 and choose the same bit rate as the MP3 that you are importing.

Yes this is lossy (you are degrading the MP3 further by re-encoding it). See here for other “direct MP3 editing” programs you can use to change the volume without re-encoding the file:
http://wiki.audacityteam.org/wiki/Lame_Installation#lossy .