It comes down to a simple theorem – the larger the tile, the greater the mass AND the greater the mass, the greater the expansion and contraction of the tile because of exposure to the sun, direct UV exposure and other atmospheric conditions. The other way to think about this is – the smaller the tile, the less the expansion and contraction and, consequently, the safer the tile in an exterior submerged environment. Our size requirement recommendations are based on limiting the amount a tile may expand and contract. Please note, we classify submerged applications and water features in the same usage category. However, showers are not in this same category because the space is intermittently wet/dry.