Regarding the hole size, opposing forces at work here. Last night I had a hard time opening a jar of pickles that was in the refrigerator so I ran hot water over the cap and it twisted off easily. The heated metal in the cap expanded in all directions. One result of the expansion was that the circumference of the cap increased resulting in a bigger hole. The circumference was able to grow because the expansion displaced air that provided little or no resistance.
As an experiment, lets embed the outside of the cap in concrete prior to heating it up (assuming that concrete does not expand in the relevant heat range). Upon heating the cap, the metal would still expand but the expansion would be constrained by the resistance of the concrete. Therefore, the expansion would be directed inward, resulting in a smaller hole (and the cap may go out of round).
Regarding a spark plug hole, an aluminum head is not like either air or concrete. It is metal that expands and is malleable. The notes to Lecture 21, Pre Flight in the referenced web site is cute but is intended to highlight a difference in the relative rate of expansion in aluminum versus copper. I would not bet the farm that it provides definitive guidance that a spark plug hole would either expand or contract.