I have used
ffmpeg to concatenate two MKV containers with both files having AV1 codec like so:
ffmpeg -f concat -i input.txt -c copy "out.mkv"
Interestingly enough, the first file has 12GiB and the second has 15GiB, but the concatenated result has only 19GiB (70%). Do MKV containers or videos in general have such a high overhead or did something go wrong?