The biggest factor is the rod ratio. Basically its the length of the rod compared to the stroke of the engine. The shorter the rod ratio, the more stress is put on the assembly because at BDC, the rod is almost pointing to the side of the bore. With a longer rod, the angle is alot less. The less angle, the less side stress is placed on the moving parts. This will decrease wear on the bore, piston, ring, bearings etc. I forgot what most engine builders shoot for. I believe its like 1.7:1 or so.