Those are The Twin Towers on Pigman’s chest, with the red S making it read September 11. This being a Capitalist Pigman, it also makes out a dollar sign. As well as evoking Superman's S.
In addition, and not as obvious, the part of the S that goes off the left side makes out a 9, making it 9/11 as well. And further, the two small dents on the right tower are meant to make out a number 3, making it a 93, which is an homage to the heroes of flight 93, but can also, unintended on my part, reference the first jihad attack on the towers in 1993.