To tell which fraction is larger, we need to understand which one is "missing" less.
For example, we have 3/4 and 7/8.
3/4 misses 1/4.
7/8 misses 1/8.
According to the rule, we choose 7/8 as a larger one because it "misses" less then 3/4. (1/8 < 1/4)