Anyone who knows Simpsons knows that Ned Flanders is very muscular. So muscular, in fact, that when he takes his shirt off his chest is much larger than his abdomen. What's strange is that when he puts on is normal green sweater, he suddenly gets a gut. With a sweater his stomach sticks out more than his chest. How odd.......
It's part of the humor. "The man with the chest" and Willie obviously don't look extremely muscular in everyday life, but that is what makes it more humorous when they take off their shirt. For example, when Willie rips off his shirt to wrestle the wolf or any other scene.