As part of the exposition, John Steinbeck directly and indirectly characterizes his two key players, George and Lennie, extracting right away the traits that make them so dramatically different.
The first thing we know is that Lennie is George's "huge companion". Having established that the man is so large, Steinbeck juxtaposes this description to the actions that this big man does, which make him appear to be acting like a child.
[HE] flung himself down and drank from the surface of the green pool; drank with long gulps, snorting into the water like a horse.
We also notice the way that George treats Lennie. He is rough with him, patronizing, and clearly seems to be the alpha male of the relationship. Much to the initial shock of the reader, Lennie accepts and even welcomes this treatment, to a point. This is the leading action that takes us to the reason why the men are there in the first place: Lennie's brute actions got the men in trouble in their former location. It is now clear to the reader that Lennie is cognitively impaired and that part of his problem is his inability to control himself.
Perhaps the most shocking thing we find out about Lennie is that his body strength is proportional to his emotional acumen. He is extremely sentimental and tries to express his emotions physically. However, he cannot control one nor the other, so he ends up hurting--or killing--that which is his object of affection.
For example, in this chapter, Lennie had brought a small mouse with him. When trying to pet it, he killed it. He has done this before and George reminds him how he much rather not have any for fear of killing whatever he touches, like it happened back when his aunt use to give him mice as pets. Lennie does not even remember this.
George scoffed. "Lady, huh? Don't even remember who that lady was. That was your own Aunt Clara. An' she stopped givin' 'em to ya. You always killed 'em."
Therefore, the strange behavior, so dissonant with his size and threatening look, his immense strength, his lack of intelligence, and his dependence to George are the strangest and most salient traits we can get from Lennie in chapter 1.