I don't know if i can explain it very well but the reason that the hole will grow larger rather than smaller is that when you heat it up, the metal expands. If you think about the inside of the hole, if the diameter gets smaller, you are actually shrinking the metal.
When heated the molecules will have more kinetic energy and need more room to resonate. or whatever Al alloy molecules like to do with their energy. If you think of the molecules that make up the filler plug hole as a bunch of people all crowded together holding hands in a circle, if they all move in towards the centre of the circle they get more squashed and uncomfortable, which would happen if you cooled the gearbox. If the people in the circle start dancing around and waving their arms and doing star jumps, they will have to increase the size of their circle, which is pretty much what happens when you heat the gearbox housing around the filler plug.
Sorry for a pretty crappy analogy and if you don't get my drift I do understand. Just TRUST me and others who know that it works!