I just so happen to make Automotive pistons for a major manufacturer, but I can still only guess.

All of our pistons have to be heat treated before machining. They are put in "age ovens" which bring the pistons up to just under 500 degrees (I think) and cooled. If they are not heat treated properly they will warp and react otherwise unpredictably when machined and when heated.

Auto pistons have ridiculously tight tolerances and require specialized CNC's to make (the pin holes for example are +2.5 microns/-1.0 micron: a micron is 1/25th of a thousandth of an inch), but I wouldn't know about these little guys. I would say that if a model engine piston wasn't made using the proper controls it could very well grow. (Ford just sent us back 16,000 pistons for this very reason.) But if it was made right, it wouldn't.