Few comments... First steel and cast iron grow at almost the same rate (actually if anything cast iron grows slightly less per degree F then steel does).

Aluminum definately grows more per Degree F then steel. This is why you'll see on an aluminum headed motor, that they will have a bigger cold/hot change, then an iron headed motor.

Iron block/iron head will have almost no change at all in hot/cold lash (maybe .002")

Also, I think the reason that you see a hot change at all is because the head itself sees more heat then the rest of the parts do. The pushrod certainly doesn't see the combustion flame front that the head does. More heat=more expansion.

Here is a chart for some comparison...

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html


As for the hypereutectic piston question. The KB hypereutectic pistons are made of a special material. That material has a slower thermal expansion, along with a lower heat transfer. That means if the piston is seeing (absorbing) less heat, all of the other components take more heat (especially the top ring). Because the top ring sees more heat, it needs more gap. As commented on before, moving the top ring up helps keep the combustion gasses in the chamber. Also, KB likes to keep the 1st/2nd ring as far apart from each other as possible. Their theory is that pressure build up between the two rings can cause ring flutter. They like big gaps, and big volume between the two...