Following is a look at how the depth of the stone influences the cushion cut.
This type of diamond looks very small in relation to its carat weight. If the diamond were too deep, you would not be able to show it off to good effect. The weight would be mostly distributed towards the bottom, but the opposite is what you want for a bigger look. For instance, a 2 carat stone could end up looking the same size as a 1.25-carat diamond.
This type of diamond appears to have a dead spot right in its center, thanks to light getting out through the bottom instead of reflecting out the top the way it should. The best diamond would have the right proportions to facilitate entry of light into the stone, as well a reflection backs out the top, delivering a brilliant sparkle.