MKVToolNix: Complete m2ts-to-mkv reduces file size - why?

[RogueComet]

A technical issue I'd simply like explained. When re-packing complete bluray m2ts-streams inside mkv-files, I've noticed significant reductions in file size. Say MKVMergeGUI lists the contents of a m2ts-file like this:

1) A VIDEO STREAM
2) AN AUDIO STREAM
3) A PGS-SUBTITLE STREAM

Now I check ALL and have the file repacked as mkv. The finished mkv will be significantly smaller - I'm talking of more than 1 GB when the source is a typical BD50-sized m2ts. Why is this? Since I've grabbed all streams and no actual compression is applied, what data has been lost to explain the smaller file size?

[El Heggunte]

The CONTAINER OVERHEAD of a transport stream is much greater than the container overhead of other formats (MPG, AVI, MKV, MP4). In this way, when you remux from (m2)ts to Matroska, you will ALWAYS get a very-noticeable filesize reduction.