Why does Curley pick a fight with Lennie, the strongest of men?
The most elemental reason that Curley picks the fight with Lennie is to prove he can. The perception of Curley being "yella as a frog belly" and publicly shamed eats away at him. Slim's challenging of Curley in such a demonstrative manner has caused Curley to be even more sensitive. When he sees Lennie smiling at the thought of the farm and the rabbits that George has discussed, Curley sees it as Lennie laughing at him. This causes Curley to fly into a rage and take on Lennie.
It never dawns on Curley to refrain from attacking Lennie because of his size. Rather, Curley recognizes fairly quickly that the bigger the target, the better it is for him. Candy alludes to this in some of his first interactions with Lennie and George. Curley believes that the bigger the target, the greater the chance for his own name to increase. If Curley defeats the bigger target, he is seen as a fighter. If he loses, the bigger adversary will receive the blame for exercising his strength against such a smaller opponent. Either way, Curley emerges as victorious. The intensity of the moment is what compels him to attack Lennie, but this dynamic also operates as to why he would attack the bigger Lennie.