Muliplexing MP4 & SRT to MKV results in smaller file size?

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1

Hi

If I use MKVToolNix to multiplex this MP4 and this SRT to an MKV, why is the resulting file size roughly 2 MB smaller?

I would have assumed that adding the SRT to the MP4 video and audio streams would result in the same files size (or 93kB larger). This ~2MB shrinking has happened to all the 4-5 files I have tested this scenario on.

I have the Compression box set to “no extra compression”. Is this tool compressing the video and audio streams anyway? I thought the point was to just reassemble the streams with the SRT without reencoding? Where does that 2.1 MB go?

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2

Welcome!

This is normal. Different container formats have different amounts of overhead, solely for the container. This has nothing to do with modifications to the content (meaning the compressed audio/video is not compressed further).