Gravity is universal, and we spend our daily lives resisting its influence. While walking, standing on one leg, or tipping back in a chair, we experience its effect and intuitively seek a state of balance.. When we are off balance, we have a strong fear that gravity will pull us over and we will fall down. Those expectations are so strongly ingrained in our subconscious that they also have an effect on the art we experience and produce. Most artwork is viewed in an upright orientation - in terms of top, sides, and bottom. as a result, gravity effects the visual composition.
Do you know why we frame our art? Artists often mat their work to gain an "aesthetic distance" or separation from the every day world. The hope is that we will see the work in a new context. But, even here, psychological factors can affect the visual weight and balance. In the case of a mat with two inch top, sides,and bottom, the bottom may have the illusion of being pinched or smaller than the other sides. This is an optical illusion that would make the artwork appear to be unstable, even rising on the wall. To compensate, the bottom measurement is generally made wider that that of the top and sides so that the whole rendering seems stabilized or balanced.