While Thornton is much bigger, remember that Drew Doughty was in the media talking about how they wanted to knock the Sharks out of the playoffs before they even start. As the captain, Jumbo probably felt it was his job to shut Doughty up, or at least show him how tough the Sharks were gonna be in these 2 games. It wasn't a cheap win, it was doing exactly what the team needed.
Thornton rains lefts all over the Human Mop... He could have picked him up and dusted the boards at the Kings bench... The most active thing Doughty did was after the fact, when he apparently thought he was in a mosh pit and did a couple hair flips...
Back in his salad days, Jumbo Joe could throw quite effectively. He became the "player who was too valuable to fight" (see Chara, Zdeno). And justifiably so.
But he's still got it- at least against people a weight class below him.