A domino can knock over another domino about 50% larger than itself. A chain of dominos of increasing size makes a kind of mechanical chain reaction that starts with a tiny push and knocks down an impressively large domino.
Original idea by Lorne Whitehead, American Journal of Physics, Vol. 51, page 182 (1983).
See http://arxiv.org/abs/physics/0401018 for a sophisticated discussion of the physics.
One thing not discussed in the video (but the paper treats in gory detail) is that it’s not just an energy argument — you also must consider the collision between the dominos. It should be pretty obvious that the dominos can’t be separated by more than their height, else they won’t collide, but that they also need to hit the next high enough in order to exert a sufficient torque to topple it. Which contributes to this limit on the size of the next one.
I don’t think his math works out. He says the final domino releases 2 billion times the energy of the first. Well, if we assume each domino is 1.5 times as large in the length dimension, and they are all made of the same material, then their masses should scale with the cube of the length. Their CG height should scale with the length, so the gravitational energy should scale with length^4.
(1.5^12)^4 is about 280 million. The sum of the energies of all dominoes is 350 million times the first domino.