That is why high silicon aluminum is used for pistons, often over 22%.
The high silicon aluminum does expand less than most other aluminum alloys in fact all but beryllium alloyed aluminum. Most silicon aluminum alloy's have a slightly higer expansion rate than most copper or brass alloy's. The actual alloy's used still require the aluminum piston to expand slightly more than the brass sleeve.
It provides better wear resistance than steel too because of that, and the coefficient of expansion is lowered from standard aluminum alloys.
Silicon is harder than steel, but that does not necessarily means it wears better. In fact at very high temps the alloy will break down causing some silicon to come out of solution and act as an abrasive. The looser clearance and thicker oil used (100% castor) can result in all iron engines to last much longer than an ABC engine.