Exporting to Mono MP3 with smaller size

I’m recording some audio for a phone voice menuing system. Since there is no advantage of recording in stereo for this purpose, I ticked “mono downmix” on the export screen.

Works fine BUT, the exported files are the same size as the stereo exports. Since the file only contains half as much information, shouldn’t it be half the size?

This is a concern, as the system has a size limit of 5MB (and a couple branches of the menu contain long messages).

Anyone know what’s going on here? Thanks.

If there’s no stereo information in these files, it’s possible that the encoder is automatically using joint stereo mode because of the low bitrate. This sums all of the audio information that’s identical in both channels.