To "pig out" is to eat a great deal of food, thus resembling a hungry pig.
Pigs do not have table manners, so the phrase implies eating without any regard for manners whatsoever. Either way, the phrase implies eating a large amount of food.
Example: "Martha's kids were pigging out at McDonald's the other day. They were eating so many French fries, I thought they would burst." This means, the children were eating a great deal of French fries. French fries from a fast food restaurant are usually eaten with fingers, not forks, so "pigging out" can be an appropriate idiom for eating that way.