He had a huge shit-eating grin after winning the game.
shit-eating grin
idiomA broad smile indicating self-awareness that may suggest self-satisfaction, smugness, discomfort, or embarrassment.
She wore a classic shit-eating grin when she realized her prank had actually worked on her brother.
Despite the gravity of the situation, he could not suppress a shit-eating grin, clearly enjoying the discomfort he had caused his rivals.