The deeper you put your spoon in the cup, the bigger swirls you get when you stir it. Similarly, created air circulation is stronger if the body of fan is thicker, height thickness are 4cm, 9cm, 12cm, 16cm. Please see comparison chart below :-

Invisible series WL-4 / WL-4-1

Classic series WL-9

Classic series WL-12

Maximim airflow series WL-16