At the beginning of the novel, descriptions are provided. If I remembered correctly, George is small whereas Lennie is a big guy and has large hands. Although George is small, he's like the father figure and Lennie looks up to him. George is the one who directs Lennie towards the right path.
An example from the novel that proves that is when Lennie was checking Curley's Wife out. George warned him to not associate with her.
