Size and Shape constancy reduce the number of changes that must be decoded in our environment.
It is believed that some schizophrenic children lack this perceptual process and do experience a frightening world in which people and animals swell and shrink before their very eyes.
Size Constancy, in summary, reduces the factor of change in our perceived world and hence simplifies the perceptual process.
Shape Constancy also contributes to this simplification, which if you remember, is a form of internally generated redundancy.
Look at any tabletop and describe it. Usually we will call it rectangular. But is it really? It is rarely rectangular, as far as our retina is concerned.
Depending on our angle of view (and there can a different one for each step we take around the table), there will be a different polygon shape portrayed on our retina, none of which will be a rectangle. Our brain, however, makes the necessary corrections for us and we perceive a rectangle.