Golding uses very interesting characterization when it comes to Piggy in Lord of the Flies. Roger perceives Piggy as a "bag of fat." Ralph originally learns of Piggy's nickname from Piggy himself, and much to Piggy's chagrin uses the moniker in the first meeting of the boys on the island. The nickname sticks, and the reader never learns of Piggy's real name--even though names do play a significant role on the island. Piggy's collecting and learning the names of the boys was one of his ways of maintaining order.
The strongest element of Piggy's characterization comes in the natural contrast that Golding provides between Piggy and the real pigs on the island. The real pigs on the island are also referred to as "bags of fat," and the boys in the end of the novel transition from killing pigs as wild game to the murder of Piggy, who is killed in a very savage fashion by Roger. The only boy on the island to wear glasses, Piggy is often portrayed by Golding as squinting or being squinty-eyed, which is similar to the small beady eyes of the pigs on the island.
Ultimately, Piggy's animal characteristics are set off by his strong intellect and scientific reasoning, his true strength on the island.