Solution:

When comparing sizes, if there is a big difference between the two items, you can directly draw a conclusion just by observing. For example, look at groups 1, 2, and 3, it's obvious the watermelon above is bigger, the pencil above is bigger, and the dog below is bigger.

When two items look about the same size, and you can't tell by simply looking, then you can overlap them to compare.

Solution:

The red fish is bigger than the yellow fish, which means the yellow fish is smaller. So in the first box, color the smaller fish yellow and the bigger fish red. The red fish is smaller than the blue fish, which means the blue fish is bigger. So in the second box, color the smaller fish red and the bigger fish blue. Finally, compare the sizes of the fish in the two boxes. You can tell right away that the blue fish is the biggest and the yellow fish is the smallest.