The potato does not swell due to the fact that it is only part of the plant. The water in the ground is taken up via the rest of the roots and transferred to the leaves and other parts. The potato itself does not exchange water with the soil.
Wet climates obviously have more water than driers climates. Metals, when subject to water have the tendency to oxidize (rust). Therefore, if water is barely present in a climate, then oxidation is also least likely to occur.
