Little is known about Piggy's background other than his quick response in Chapter One to Ralph's criticism that Piggy swims badly, and the fact that his father is in the Navy and has taught him to swim. Piggy tells Ralph that his father is dead and his "mum" must have left as he breaks off when he mentions her:
"My dad's dead," he said quickly, "and my mum--"
"I used to live with my auntie. She kept a candy store. I used to get ever so many candies. As many as I liked. When'll your dad rescue us?"
His words indicate that Piggy is rather insecure. For, there is no mention of any kind of interaction between the aunt and him other than her being lenient and allowing him to eat as much candy as he wanted--perhaps, to keep him from bothering her, or out of pity for him. He indicates this insecurity when he and Ralph discuss where they are in Chapter One:
"I climbed a rock," said Ralph slowly, "and I think this is an island."
"They're all dead," said Piggy, an' this is an island. Nobody don't know we're here. Your dad don't know, nobody don't know--"
Piggy's poor grammar indicates that his social background is certainly not of the upper middle or upper class since studies have shown that basic language patterns are formed by age five or six are learned at home. However, he is part of the group of school boys, so his "auntie" must have enough financial resources to send him to school.
In his solitary existence, then, Piggy may have had to fend for himself and become more adult-like sooner than other boys. At any rate, he is more mature in appearance and thought than the others. And, he represents the adult-like rationality in Golding's allegory.
The lines mwestwood gave you are about the only certain things we can know about Piggy. The rest we have to surmise.
His parents are not in the picture, which is why he's living with an aunt. That aunt is quite lenient, even overindulgent, and Piggy is allowed--even encouraged--to eat all the candy he wants. That makes the aunt culpable, in part, for Piggy's size. She may have been acting out of love, but she clearly did him no favors.
He is asthmatic, oversize, and bespectacled--all of which would cause him to be an outcast in any school, and especially an all-boys' school. His nickname--Piggy--was a source of pain for him.
"I don't care what they call me...so long as they don[t call me what they used to call me at school" (ch. 1).
Clearly he was picked on before he arrived on the island.
In chapter 5, Piggy shows a bit of a scientific bent, mentioning space exploration (Mars, specifically) and chiming in on the discussion about the giant squid as a mythical beast rather than a real creature.
Piggy is the one who takes the time to listen, and the littluns do seem to confide in him--indicating he has spent some time with younger boys somewhere. He bends down, on their level, to talk to them, something no other boy on the island does for the youngest boys. It probably comes from experience.
In this symbolic novel, Piggy clearly represents the intellect, and he's probably developed that skill because he hasn't had the distractions of friends, sports, family or social activities to get in the way. We all love Piggy because he's what we all want to be (wise) but hope we aren't (overly intellectual and socially inept).