Mass has nothing to with how much expansion a material has. Expansion is based on the length of the object, the co-efficient of expansion for that material & the temperature difference.
The C of E for steel is 0.000012 [ for degrees C ].
Take a 9" prod. The change in length going from 68*f to 150*f is 0.005". Because it is the longest part in the valve train, a small temp change variation can result in a couple of thousandths in length; & because it is on the prod side of the engine, any change in lash has to be multiplied by the rocker ratio.
It is easy to see why some combos vary in the difference between hot & cold lash & there is no hard & fast rule.
