changing Quality Slider doesn't change file size

I had a MP3 file I wanted to clip off the end couple of seconds. I loaded it into Audacity 2.1.2, clipped off the end, then exported it. I chose AAC as the export format. At the bottom was a slider called Quality. I exported twice, with the Quality Slider set to 255 and to 128. Both times the resultant file came out to be 5.5 MB. Shouldn’t a lower quality file be smaller?

Sorry, the slider does not work with the FFmpeg version that we recommend, as described in the 2.1.2 Release Notes. There are workarounds mentioned in that link.