I already removed my not needed audio and subtitle files when ripping Blu-Ray Disc. I use MKVToolNix to remove chapters also.
Original file size before MKVToolNix is 9,847 MB, but after removing chapters or not, doesn’t matter after multiplexing, program adds 21 MB and it becomes as 9,868 MB. It was only an example, it also makes this same thing on other video files too.
I use default settings not changing anything. MKV to MKV. How do I fix it?
Matroska is a very flexible file format with multiple optional parts and several decisions to make for tool implementers, and all of those can impact files sizes in small ways. There’s no way to say exactly why this happens in your particular instance without taking a very close look at both the file original & the new file. Therefore there’s no way to make recommendations how you could reduce the file size again, and what the drawbacks would be.
mkvmerge makes very sensible decisions by default. There’s nothing to fix, so don’t worry about a size difference that isn’t even 1/3 of 1% of the total file size.
MKVToolNix does not include decoders or encoders. It cannot reduce quality. It solely operates on the container level.
If your players support zlib-compressed data, then leave it on. It’ll save a bit of space, mostly for image-based subtitle types.