That is not a smirk! It is a strained smile.
Just a typical Romney smile.
I think you're right. It's not a smirk, smug and self-satisfied. It's also not a happy or joyful smile.
It's hard to tell since you can't see more than half his face but I'd say it's the expression the face relaxes into after you've had to do something a little difficult and have got it behind you.
You don't want to look exhausted or numbed, especially if you're a politician with the camera's on you, so your face relaxes into a little smile.
Maybe this is more like a smirk:
This Romney thing is hard to fathom: a barely perceptible micro-expression somehow taken as revealing something deep about the man, but people do tend to read a lot into politicians depending on whether they support or oppose them. If someone is ready to believe a President of the other party has killed dozens of people here at home who could cause him trouble, the "arrogant, smirking" thing shouldn't come as a surprise. And "the Bush smirk" did work against Bush. Maybe not in the reelection campaign, but certainly by the end of his term.