The cut of a diamond determines its brilliance. There is no single measurement of a diamond that defines its cut, but rather a collection of measurements and observations that determine the relationship between a diamond's light performance, dimensions and finish. Most gemologists consider cut the most important diamond characteristic because even if a diamond has perfect color and clarity, a diamond with a poor cut will have dulled brilliance.
The width and depth can have an effect on how light travels within the diamond, and how it exits in the form of brilliance.
Too Shallow: Light is lost out the bottom causing the diamond to lose brilliance.
Too Deep: Light escapes out the sides causing the diamond to appear dark and dull.