Different file size after changing container format

[gx240]

I was looking for a way to convert .ts files to .mkv without modifying/re-encoding the audio and video, and a quick Google search told me that MKVToolNix is ideal for doing this. I just tested it by converting a few videos with h264 video and aac audio in .ts containers to mkv.

It seems like everything worked fine, and the process was nearly instant. However I notice that there is a difference in file size after conversion. The new mkv files seem to be about 5-10% smaller than the ts originals. I was under the impression that the container format contributed little or nothing to the overall file size, so changing it should have little effect. Is that incorrect, or have I lost something else in the conversion process? I don't think I've inadvertently re-encoded or modified the video/audio in any way. (I only ask because I'm unfamiliar with this software.) Re-encoding usually takes a noticeable amount of time and system resources, right?

[nevcairiel]

TS files can contain (useless) filler data to achieve a certain fixed bitrate. MKVToolNix removes this filler data, which is why the resulting MKV files are a few percent smaller.
Additionally, TS has a bigger overhead than MKV, so its expected for files to be smaller - how much I cannot say for sure.