* MKV Structure: MKV files are containers that hold various streams (video, audio, subtitles) separately. They don't actually merge these streams into one entity.
* Subtitles as Separate Streams: Subtitles are stored as separate streams within the MKV container. To be "mixed" in, they need to be added to the container.
* Re-encoding and Size: Adding a new stream to the container requires the video player to re-encode the video and audio streams, resulting in a slightly larger file size.
Alternatives to Avoid Significant Size Increase:
1. External Subtitle Files: The most common and efficient method. You can store the subtitles as a separate file (.srt, .ass, etc.) and use a media player that supports external subtitles. This keeps the MKV file size unchanged.
2. Soft Subtitles: Some players allow "soft subtitles" that are embedded into the video but are not permanently part of the video stream. These can be turned on or off as needed. However, this usually results in a small increase in file size.
3. Muxing with a Minimal Change: You can use tools like MKVToolNix to "mux" the subtitle file into the MKV container. This won't re-encode the video/audio streams, but it will still add a small overhead to the file size due to the additional subtitle data.
Recommendation:
Using external subtitle files (.srt) is the best option for keeping the MKV file size unchanged and having flexibility in choosing which subtitles to display. Most modern media players support external subtitles, and you can find them easily online.