What is key to the presentation of both George and Lennie in Chapter 1 is the animal imagery that Steinbeck uses to describe them, and, more importantly, the relationship between them. Consider the first description we are given of their differences:
The first man was small and quick, dark of face, with restless eyes and sharp, strong features. Every part of him was defined: small, strong hands, slender arms, a thin and bony nose. Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, with wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags his paws. His arms did not swing at his sides, but hung loosely.
Firstly, the fact that George leads the way is highly significant. Lennie is a character who always follows and never leads. The description of Lennie as a bear indicates both his incredible strength, but also the low level of intelligence that he has. These impressions are confirmed as the chapter progresses. For example, when Lennie throws himself down to drink from a pool with "long gulps, snorting into the water like a horse" it is George who has to stop him from making himself sick. Also note the use of animal imagery again to describe Lennie. Steinbeck refers to Lennie's hands as "paws" in this chapter, reinforcing the bear-like imagery, and always we see George in the role of protector and carer of Lennie - although he is smaller than George, it is he who has the brains.