Both Lennie and George are wearing clothing appropriate for traveling ranch workers: denim, black hats, and blankets to sleep on - wherever they happen to rest. The physical descriptions of each man fit their respective psyches and personalities.
George is defined as "small and quick." He has restless eyes and everything about him is "defined." He is thin, small, and strong. George has a strong, commanding personality. He is direct and quick with that directness. His eyes are restless because he doesn't trust anyone and he must always be on the lookout for Lennie's well-being. As much as he is Lennie's companion, he is also like a parent watching over an unintentionally troublesome child. As such, his eyes are always scanning the area, watching Lennie, and watching out for others.
Lennie is George's opposite in personality and physical description. Lennie is huge, shapeless (undefined), with slopping shoulders. He walks like a bear. Whereas George is quick and discerning, Lennie is slow and oblivious. His arms hang but do not swing at his sides. In other words, he does not walk with confidence, purpose, and determination. George does. George is confident and purposeful with his movements. Lennie is unsure and naive. There is a wisdom to George's appearance and movements. There is a childish lack of awareness in Lennie's behavior.