Table width is quoted as a percentage of the average width of the diamond's girdle. Table size is less critical to the beauty of a diamond than variations in the crown and especially pavilion angle. AGS and GIA's new systems each have top grades that include 61% table sizes.
Yields from most rough diamonds are improved by polishing larger table sizes than we might prefer. Small variances in crown and pavilion angles cause less problems if table sizes are in the range of 55% to 58%.
Larger table diamonds have a better "spread" and can be brighter, i.e. they return more light, but are often less firey than diamonds with smaller tables. They often have less scintillation. Larger table sizes are considered to be an advantage in smaller diamonds below .10 ct.