Strength of dominoes.

I just took a look at an experimental joint I did a couple of days ago where I used some dominoes on some 12mm MDF. The Titebond glue has obviously done its job and with the dominoes, the two offcuts I joined are very, very rigid. That got me thinking about the dominoes and what it is that makes them so strong.

That is, I also have a Dewalt biscuiter. I noticed that the beech biscuits absorbed water if I left a few overnight in a glass of water and hence they expanded. If I used beech biscuits too close to the outer surface of the material, I could sometimes see that the biscuit had "telegraphed" its shape to the outer surface. I then did the same with the dominoes on the 12mm MDF. My experimental cuts with the Domino were perhaps 3mm from the outside surface. I glued the edges and put enough glue in the slots and then clamped the MDF to set.

Even though I was about 3mm from the surface, nowhere have any of the eight or so 5mm dominoes I used "telegraphed" their presence to the outer surface. I also dropped a few dominoes into some water and the following morning they still seem to have retained their original shapes unlike the DeWalt biscuits.

Do the dominoes actually expand in the presence of glue to give them their mechanical strength or is it the precision of the depth cut and the fine tolerance that gives them their strength? Probably a combination of both, I'm sure, but I'd like to read your opinions about it.