I guess I should probably try and handle this one.
A pig is obviously someone who can't get enough of something, and it sort of applies that way in the gay market as well. But depending how it's used, it tends to take on a dirtier connotation.
For instance, a cum pig would usually refer to someone into bareback sex who would take load after load in either orifice. A cock pig would be someone who sits at a glory hole and sucks one cock after another.
Piggy sex refers to sex that is nastier and dirtier, so spitting, pissing, stinky arm pits, forcing your cock down a guy's throat, making him gag, spitting in his mouth or on his face, not washing under your foreskin before sex ... raunchier sex.
Where bears and twinks refer to the look of a person, a pig does not. Pig refers more to the activities one would engage in.
Cheers
Michael