William Golding allows both the main protagonist and the main antagonist of the story to take part in mistreating Piggy. Although Jack is by far the worst offender, even Ralph, who is normally a kind and principled person, treats Piggy cruelly at times.
The first two characters readers meet are Ralph and Piggy. Piggy asks Ralph his name and expects Ralph to politely reciprocate. Piggy starts hinting to Ralph to ask him his name and, in doing so, reveals his hated nickname. Immediately, "Ralph shrieked with laughter." He cavorts around, repeating the name and laughing. Piggy takes it in stride but begs Ralph not to tell the others. When the group convenes, Ralph says in front of them all, "He's not Fatty . . . His real name's Piggy!" Piggy confronts him to let Ralph know he hurt him. Ralph is tempted to double down on the insults but instead gives Piggy the job of taking names. He doesn't allow Piggy to come on the trek around the island, thereby excluding Piggy from the in-crowd, but he does assign him a role. Later in the book, Ralph becomes more respectful toward Piggy and comes to value his advice and loyalty.
Jack is cruel to Piggy from beginning to end. At the first meeting, he tells him that he's talking too much and says, "Shut up, Fatty." Several times in the novel, he makes unkind remarks directly to Piggy, and he makes digs at Ralph for protecting Piggy. When Piggy scolds Jack for letting the fire go out, Jack "stuck his fist into Piggy's stomach." He follows that up with a slap to Piggy's head, which causes Piggy's glasses to fly off and break. Instead of apologizing, he mocks Piggy's whine. He pantomimes Piggy's scramble after his glasses, causing the other boys to laugh at Piggy's loss. At the first pig roast, Jack intends to withhold meat from Piggy, but Simon gives his piece of meat to him. Later, Jack conducts a midnight raid and steals Piggy's glasses, leaving him all but blind. Finally, Jack argues with Piggy at Castle Rock at the confrontation that leads to Piggy's murder.
Both Ralph and Jack mistreat Piggy, but Jack's treatment of Piggy is exponentially worse than Ralph's.