Domesticated pigs are fatter not just because they are fed more but also because they were bred to be, same as cows.
I read the aborigines had a custom where they knew when all their favorite fat sources were available at different times of the year, pine nut harvest, fattiest kangaroos, fattiest clams, etc. When the kangaroos were at their fattiest they hunted them to excess to extract the limited amount of fat from their bodies, sacrificing large amounts of meat in order to enrich their diet with fat.