In chapter one of "Lord of the Flies," Piggy becomes angry that Ralph has let the other children know that his former nickname was "piggy" because he is heavy-set. He feels that Ralph betrayed him by revealing this information that was spoken in confidence. Piggy was hoping to avoid the unpleasant nickname, and in retrospect, probably should have never mentioned it.
Ralph is not very sympathetic. His opinion is that "piggy" is a better nickname than some of the other ones they could have come up with, like "fatty." This is the first instance during which it is shown that despite his intelligence Piggy will get little respect from his island-mates.
In addition, he is angry that Ralph does not choose him to be one of the boys who gets to explore the mountain, instead making piggy stay back to work on a census.
well piggy tels ralph not to tell the others that his name is piggy but ralph does... does this help??