Earth's size is a major reason for its unique surface, compared to other bodies like the moon, Mercury or Mars. Earth's surface-volume ratio is small enough for its crust to break into plates and it is big enough to generate the level of internal heat required to move these plates around.
Moon and Mercury cannot generate the necessary internal heat to have an active surface. What about Mars? Mars' surface might represent an unsuccessful try at plate tectonics; its crust fractured but could not move.
Pascal once remarked, knowledge is like a sphere in space. the more we learn (volume of the sphere), the greater our contact with the unknown (surface of the sphere). True enough - but considering the surface-volume relationship - the larger the sphere, the greater the ratio of known (volume) to unknown (surface)!
Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould
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