The smaller denominator will give a larger fraction. Another method for comparing is to think of the fractions. In this example it is obvious that 1/3 is smaller than 7/8. For one thing, 1/3 is smaller than 1/2 and 7/8 is larger than 1/2.
so therefore 2/3 is not greater than 7/8
2/3 only equal .66
7/8 equals .875