I think Golding uses the word "enormity" when describing the scene with the pig in order to show how big of a deal it was. All the boys were hungry and they needed food, and the oig would have been their first meal. Jack was about to kill the pig, but he hesitated just lonbg enough so that he didn't have to. This shows that he really is just a kid, and that they will have to adapt to survive. This shows what they are going to have to do to eat, and also how big of a deal it was that Jack let the pig get away. Jack was ready to kill the pig, but when it came down to it, he could't.
I agree with Victoria's explanation. The word "enormity" was used in that scene to describe the largeness of the situation, and its big impact. As Victoria previously said, the boys were all hungry and the pig would have been their first meal. Although Jack acted like he was going to brutually kill the animal, in the end he was reluctant and was afraid of the enormity of his knife cutting into living flesh.