Small-diameter round steel (<20 mm) costs more per ton than larger diameters (>100 mm) due to higher processing costs. Cold-drawn small bars require multiple passes, increasing labor and energy use-e.g., 10 mm cold-drawn 1018 may cost 800/ton vs. 600/ton for 100 mm hot-rolled. Larger bars have lower unit costs due to economies of scale in hot rolling. However, specialty large diameters (>200 mm) for heavy machinery may cost more due to limited production capacity and stricter quality checks (ultrasonic testing), offsetting the size advantage.