Even better than that: the thermal time constant is proportional to the square of the radius. There's the square/cube effect that causes a factor of r ratio in the volume to surface area, but there's also a factor of r from the temperature gradient (which, from the center of the pile to the surface, is inversely proportional to r).
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Even better than that: the thermal time constant is proportional to the square of the radius. There's the square/cube effect that causes a factor of r ratio in the volume to surface area, but there's also a factor of r from the temperature gradient (which, from the center of the pile to the surface, is inversely proportional to r).