While he was a kid, he was ridiculed for being overweight and was picked on by bullies. After training in martial arts, he became much more powerful and massive in size. He later intimidated his bullies into joining his gang. He eventually became the right hand man of one of the biggest gangs in New York City but he would later control them as well. He is now the crime boss of one of the largest gangs in the world and is immensely wealthy.
Despite his rather large bulk, Kingpin is not composed of fat, but rather of very dense muscle.