Hello. I was converting a m2ts (834 mb) video file to mkv, I got 758 mb. There’s a little size difference is the quality remains the same?
Best regards, Rein 
Welcome!
MKVToolNix does not contain decoders & encoders. It cannot re-encode content & does therefore not degrade quality.
Differences in sizes are expected as different container formats have different amounts of overhead.