Why does remuxing from flv to mp4 increase file size?

[90sTV]

If it's just changing the container format, and the file remains unchanged, why would it increase in size?

[DB83]

Short Answer: No


Longer answer: It may depend on the encoding of the flv.


Even longer answer: mp4boxgui (at least my version) would not accept flv as an input for muxing. But I was able to do it with TubeDigger and the resultant mp4 was actually slightly smaller than the source. I think this program uses ffmpeg so it should be possible from the command line.

[Selur]

Remuxing flv(avc,aac) to mp4(avc,aac) might de-/increase the file size but only slightly due to different headers of the containers.
-> How much of an increase are we speaking of?

[AllisOne]

If it's just changing the container format, and the file remains unchanged, why would it increase in size?

DB83 and Selur are very experienced in this subject and expect good advices from them, but I'd like to cut in as someone whose understanding is simpler, but might help.

How did you change the container? What program did you use? Are you sure you set the audio and video codec to copy/pass-through or something similar before clicking go? Some programs especially the transcoding ones have the option to just change the container, but you need manually set copy/pass-through. If not they might re-code audio and video to codecs more suitable for specific container. Even more - they can re-code them using the same codec, but different bitrate etc. and thus making the new file much bigger/smaller.

What operation system exactly do you use?

If the file is unchanged (what does it mean actually?), why you're saying it's changed? Did you checked what's inside .flv and .mp4 (with for example MediaInfo)?

A few days ago I've spotted revived Video Container Changer:
https://www.videohelp.com/software/Video-Container-Changer

If you on Windows, give it a go. It uses quite old ffmpeg, but if you replace it with a new version from zeranoe site, it seems to work too.

[90sTV]

If it's just changing the container format, and the file remains unchanged, why would it increase in size?

DB83 and Selur are very experienced in this subject and expect good advices from them, but I'd like to cut in as someone whose understanding is simpler, but might help.

How did you change the container? What program did you use? Are you sure you set the audio and video codec to copy/pass-through or something similar before clicking go? Some programs especially the transcoding ones have the option to just change the container, but you need manually set copy/pass-through. If not they might re-code audio and video to codecs more suitable for specific container. Even more - they can re-code them using the same codec, but different bitrate etc. and thus making the new file much bigger/smaller.

What operation system exactly do you use?

If the file is unchanged (what does it mean actually?), why you're saying it's changed? Did you checked what's inside .flv and .mp4 (with for example MediaInfo)?

A few days ago I've spotted revived Video Container Changer:
https://www.videohelp.com/software/Video-Container-Changer

If you on Windows, give it a go. It uses quite old ffmpeg, but if you replace it with a new version from zeranoe site, it seems to work too.

I'm using a Mac. I used VideoProc, and it has a box that was checked next to "Auto Copy." For the explanation of that, it says, "Neither the video nor the audio needs to be re-encoded. This will be super fast and retain full video and audio quality."

[DB83]

Well mediainfo is also available for a mac so upload a text mode report of the flv and the mp4.


What you should also have stated right at the beginning is your useage of a mac. This is a Windows sub-forum and you might get more help of you were to contact a mod to get the topic moved to the mac section of the forum.


Even so, the mediainfo reports will still assist.