Why would mono be larger than stereo?

[tlmurray23]

I was going through some voice recordings to mix them down from stereo to mono to save space, but they end up larger. For example a 36.6 meg file ended up 48.4 megs. No changes were made (sample Hz, etc.) along the way: Just open, mix down, and save. Spock would say, "it doesn't seem logical."  What's up?

[SteveG_AudioMasters_]

What format were the original files in?

[tlmurray23]

Recorded using Audio Hijack 3, 256 kbps, variable bitrate, auto sample rate, MP3.

[ryclark]

And what format are you re-saving them as?

[tlmurray23]

Two screenshots attached. This first one is from a File > Open > Save As. Its estimate size is 63 megs, but it's 36.6 on my drive:

and the second is after the mix down to mono. It says 41 meg but it saves at 48, but a lot closer:

[SteveG_AudioMasters_ · Accepted solution]

If you save your file at the same bit rate, it will come out the same size... I did a quick test, and just converting an existing mono MP3 to stereo without altering anything else results in a file of identical size. Converting it back to stereo doesn't alter it either - just produces a stereo file, rather than a mono one. If you want to keep the same quality in your mono MP3 you have to save it at half the bit rate - this leaves the quality alone, but will halve the size of the file

And the reason that your sizes are not coming out the same size as the estimates is because you are using VBR - which can't tell you until it's encoded the file, and generally isn't recommended anyway. If you halve the bit rate for saving the mono conversion and use CBR, you file sizes will halve.