In Lord of the Flies, what does the reader know about Piggy's background?Do we know where or when he was born? His hobbies? Where he went to...

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).