While Kenny and Tub are nominally friends, Kenny constantly insults Tub, plays jokes on him, and uses Frank as a wedge (always looking to Frank for validation of his "humor," showing that he has insecurities). One theme suggested by this is domestic abuse: Tub seemingly has no reason to be friends with Kenny, but continues to hand around him even when he is verbally abused.
"You still on that diet?" He winked at Frank.
"What do you think? You think I like hard-boiled eggs?"
"All I can say is, it's the first diet I ever heard of where you gained weight from it."
(Wolff, "Hunters in the Snow," classicshorts.com)
This speaks to Tub's personal insecurities; he is willing to put up with the abuse as long as he believes it validates him. By the same token, Kenny believes that Tub wants to be insulted; bullies often think that their attention is in itself something to be treasured. The two play off each other, each validating the other's insecurities, until one of Kenny's jokes goes terribly wrong.