This quote demonstrates that Curley is interested in picking a fight with Lennie because he is a big guy, and Curley feels threatened by him.
Curley comes looking for his father, the boss of the ranch, and sees George and Lennie for the first time. Lennie is so huge that he makes an impression right away. Curley’s reaction is not friendly.
He glanced coldly at George and then at Lennie. His arms gradually bent at the elbows and his hands closed into fists. He stiffened and went into a slight crouch. (ch 2)
He is sizing them up. Curley is basically going into fighting mode. The word “pugnacious” means aggressive or confrontational. George describes Curley this way because he has seen guys like him before. He knows that Curley is a small-minded man that is going to make trouble for them if they are not careful.
In fact, Curley does make trouble for George and Lennie—a lot of it. Lennie gets into a fight where he crushes his hand, and Lennie accidentally kills his wife, so George has to shoot him. This quote seems to foreshadow that Curley is the beginning of the end for George and Lennie.
To explain another way that Curley is "calculating" in this episode, we can consider the situation that Curley creates by picking a fight with Lennie.
Curley stands only to gain in this conflict. He is far smaller than Lennie and so would not lose face if we were to lose a fight to the bigger man. If, on the other hand, Curley were able to beat Lennie in a fight, Curley's accomplishment would be undeniable. So, for Curley, fighting Lennie is a win-win scenario.
"Being the boss’ son and a former Golden Glove finalist, he picks fights with impunity, generally targeting larger men so that he will get praise if he bests his opponent and be seen as a martyr if he does not" (eNotes).
This is one of the "calculations" that Curley makes. The underhanded and conniving element of the scenario is combined with Curley's obvious aggressive behavior (which is entirely uncalled for and unprovoked), leading to a universal condemnation of Curley's actions.
When Slim says, "This punk sure had it comin' to him," his thought is representative of the whole group.
With a young wife as the sole female on a ranch full of men, Curley is constantly worried that she is cheating on him. Problems in the marriage are suspected and the ranch hands generally feel that Curley is out to prove his manhood by picking fights with people who cannot afford to fight with him. They may be fired for fighting with the son of the boss and so are in no position to offer a straight-forward reprimand to Curley until Curley provokes them.
Carlson only rebukes Curley only when Curley has already challenged him to a fight, saying, "I don't care if you're the best welter in the country. You come for me and I'll kick your damn head off."
Curley's training as a boxer only further proves to the ranch hands that Curley has something to prove about his manhood (something that cannot be proved by fighting) and so find that Curley's pugnacity only increases their scorn for him.
Ultimately, the phrase ("His glance was at once calculating and pugnacious.") implies that Curley knows exactly what he is doing by picking a fight with Lennie. He understands his advantages in the situation and intends to act on them with immediate aggression.