Factory high compression pistons like the two valve relief one pictured above are forged, therefore a little heavier than standard castings...your pistons are low compression, and because cast slugs given the same architecture are lighter, piston manufacturers used to cast weights in them to match the weight of factory pistons so the balancing would be in normal limits. Take them out and your cast pistons would weigh probably a couple dozen grams less and would require some weight to be taken out of the crankshaft counterweights.
