A crime boss is one who is in charge of an organization whose primary objective is the acquisition of wealth and power through illegal means. Crime bosses come in all shapes and sizes and their personal circumstances and ambition may guide them towards the type of crime boss they wish to be. Few start out as the leader of a criminal organization, and most rise through the ranks either through recognition of their own merits, or they inherit it as a member of a crime family. The different types of criminal organizations may include corporate ties, drug cartels, organized crime, or smaller gangs.
For some reason, in speculative fiction, crime bosses tend to be really fat. It is possible that size does matter and that a being of prodigious volume exhumes a sense of overwhelming power. Or maybe they just really like donuts a bit too much. Some examples of some crime boss fatties include Wilson Fisk, the "Kingpin of Crime" from Marvel Comics comic book titles as well as the 2003 Daredevil feature film, Ulysses Lugman, aka the Slug! Seriously... that's the coolest code name you can come up with for yourself. And naturally, the king of all fatties... Jabba the Hutt. In contrast, being an overweight crime boss does not necessarily mean that one is the most ruthless bastard in the galaxy. Take for example Adelai Niska from Firefly. He was slight of build and elderly, yet still had the cojones to have his own nephew killed.