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[Bug]: Using M4B generation tool results in a much bigger file than original files combined #1434

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opened 2026-04-24 23:45:20 +02:00 by adam · 3 comments
adam commented 2026-04-24 23:45:20 +02:00
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Originally created by @ReaderGuy42 on GitHub (Oct 2, 2023).

Describe the issue

The original six files of an audiobook have about 700 MB and after turning it into a M4B with ABS's M4B tool it had about 1.4 GB.

This has also happened with another book.

I've just updated to the newest version but that actually made the M4B file even bigger, now it's 1.9 GB?

Steps to reproduce the issue

  1. Choose book
  2. select M4B generation tool
  3. wait
  4. see it's a bigger file.

Audiobookshelf version

2.4.4

How are you running audiobookshelf?

Docker

Originally created by @ReaderGuy42 on GitHub (Oct 2, 2023). ### Describe the issue The original six files of an audiobook have about 700 MB and after turning it into a M4B with ABS's M4B tool it had about 1.4 GB. This has also happened with another book. I've just updated to the newest version but that actually made the M4B file even bigger, now it's 1.9 GB? ### Steps to reproduce the issue 1. Choose book 2. select M4B generation tool 3. wait 4. see it's a bigger file. ### Audiobookshelf version 2.4.4 ### How are you running audiobookshelf? Docker
adam added the bug label 2026-04-24 23:45:20 +02:00
adam closed this issue 2026-04-24 23:45:21 +02:00
adam commented 2026-04-24 23:45:22 +02:00
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@advplyr commented on GitHub (Oct 2, 2023):

Abs uses ffmpeg to generate the m4b file. Nothing has changed with that in a long time so I don't know why you would be seeing a difference in size after updating.

I'm not sure there is anything we can do here since this is a fairly basic ffmpeg command but you can share the ffprobe output of one of your mp3 files and maybe it will highlight something. You can get the ffprobe output by going to the audio tracks table on the page of the book and using the context menu to select "More Info". You will see the ffprobe button.

You can also share the number of mp3 files you are merging and if you have tried merging with any other tools and are seeing different outputs.

@advplyr commented on GitHub (Oct 2, 2023): Abs uses `ffmpeg` to generate the m4b file. Nothing has changed with that in a long time so I don't know why you would be seeing a difference in size after updating. I'm not sure there is anything we can do here since this is a fairly basic ffmpeg command but you can share the ffprobe output of one of your mp3 files and maybe it will highlight something. You can get the ffprobe output by going to the audio tracks table on the page of the book and using the context menu to select "More Info". You will see the ffprobe button. You can also share the number of mp3 files you are merging and if you have tried merging with any other tools and are seeing different outputs.
adam commented 2026-04-24 23:45:23 +02:00
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@nichwall commented on GitHub (Oct 2, 2023):

It could be due to the default bitrate being changed from 64k to 128k (or whatever the exact number was), but otherwise I don't have any ideas either

@nichwall commented on GitHub (Oct 2, 2023): It could be due to the default bitrate being changed from 64k to 128k (or whatever the exact number was), but otherwise I don't have any ideas either
adam commented 2026-04-24 23:45:23 +02:00
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@advplyr commented on GitHub (Oct 2, 2023):

Yeah that's a good point. That would make sense as to why the size increased

@advplyr commented on GitHub (Oct 2, 2023): Yeah that's a good point. That would make sense as to why the size increased
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Reference: starred/audiobookshelf#1434
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