Why does crooks react this way to Lennie?

He's rude to him because he doesn't like people that are bigger than him

"For a moment Crooks did not see him [Lennie], but on raising his eyes he stiffened and a scowl came on his face."

Crooks stiffening shows that he feels a little threatened by Lennie's presence - he is becoming defensive and ready to react to any attack Lennie might start. The scowl on his face shows he is doesn't want Lennie there and is trying to ward him off by being uninviting.

To me it also shows he doesn't want to be disturbed and just wants to be alone in his room, tending to his back and reading all his books, as this is what he is used to and has come to accept. This un-ordinary occurrence makes him suspicious as normally white men do not enter his room.

"Crooks said sharply, 'You got no right to come in my room. This here's my room. Nobody got any right in here but me.'"

This again shows he doesn't want Lennie there. Crooks shows distrust towards him.