mp4 file has turned into a 1.4GB smaller mkv file

[JamNujo]

I took an mp4 file and converted it to an mkv file using MKVToolNix GUI. The original mp4 file was 13.4GB and the new mkv file is 12GB. That's a whole 1.4GB smaller. Why is the mkv file so much smaller?

[LigH.de]

Why don't you show us MediaInfo reports of both files? Already a verbose analysis of the MP4 may give us a hint if there is wasted space to keep a nominal bitrate virtually constant.

[JamNujo]

[Attachment 49045 - Click to enlarge]
Here is a screenshot of the 2 side by side. (I did it with another pair of files because I deleted the mp4 file I originally posted about)

It seems that you are right about the bitrate. Can you explain to me what happens during the conversion from mp4 to mkv that causes this change?

[LigH.de]

I guess that the MP4 video stream was partially stuffed with useless filling data to ensure that the bitrate does not drop below a certain minimum. May happen to preserve compatibility with players which have decoding constraints. MKVmerge may have detected this stuffing and skipped it while multiplexing.

That's just my interpretation. No guarantee for being correct. Have a little patience, more opinions may arrive ...

[TimA-C]

One is showing as fixed frame rate and the other as variable frame rate which will probably have some effect. I'm guessing that the overheads of the two containers are also different which may account for some of the difference. As for the change in reported bit rate, as I understand it, not only are there various ways of measuring bit rates but also how you round and display those numbers - Kb/Mb vs KiB/MiB etc. I suspect that MKVToolnix just uses a different way of calculating and/or displaying those numbers. Have you tried running the two videos through something like 'Bitrate Viewer' or maybe GSpot?

[sneaker]

The file sizes are different. Not really any way for MediaInfo to calculate them wrong. I support LigH's theory.

[JamNujo]

[Attachment 49046 - Click to enlarge]
Here are the results side by side.

This is the result of saving the mkv file to an mp4 file with Avidemux.

[Attachment 49047 - Click to enlarge]

Could this have to do with the first file being a file created directly from a live stream? The mp4's are VODs that I downloaded at the highest quality available, which are listed as "chunked" or "source" internally.

[sneaker]

Yes, probably like LigH said. (H.264 filler NALU)