When Lennie is introduced, he is described as "a huge man, shapeless of face, with large, pale eyes, with wide, sloping shoulders ... walk[ing] heavily." This description of him immediately suggests a large animal like a bear, especially when contrasted with the description of George. George is described as "small and quick," with eyes that are "restless ... and sharp." Steinbeck's description of George suggests a thinking man, whereas, by contrast, his description of Lennie suggests an entirely physical, unthinking presence. This description is made more explicitly animalistic when Lennie is described as dragging his feet "the way a bear drags his paws." This simile suggests that, like a bear, Lennie is physically powerful, but also not particularly thoughtful. Steinbeck also writes that Lennie's arms "hung loosely" at his sides, again suggesting a bear-like appearance and movement.
A little later in chapter 1, Lennie drinks from a pool of water, "snorting ... like a horse." Steinbeck uses another simile here to compare Lennie to an animal. A horse, relative to a human, is not especially intelligent, but a horse is also a beautiful, noble creature. The implication is thus that while Lennie may not be intelligent, he is, in his own way, beautiful and noble.
The way that George speaks to Lennie is very much like the way one might speak to an animal. George uses sharp, simple, exclamatory demands, such as "Lennie! ... Lennie ... don't drink so much." The way that George speaks to Lennie serves to emphasize Steinbeck's animalistic presentation of Lennie.
Steinbeck deliberately compares Lennie to a bear, and then to a horse, mostly to emphasize that Lennie is not a thoughtful creature—but also to suggest that he is, like an animal, innocent and noble. He is perhaps closer to an animal than he is to the other men on the ranch. He is less intelligent, but more noble. He is not at all calculating, like many of the other men, and although he kills Curley's wife, he is in a sense more innocent.