KANEOHE BAY, Hawaii - President Obama may be one of the most powerful men in the world but he admits to feeling "small" when working out alongside Marines at their gym on base in Hawaii.

"The only problem I've got when I'm here is having to work out with Marines in the gym," Obama said during his annual Christmas Day visit with U.S. troops at Marine Corps Base Hawaii in Kaneohe Bay. "Because I generally feel like your commander in chief is in pretty good shape, and then I get next to some guy, you know, curling 100 pounds and it makes me feel small."

Obama works out at the base gym just about every day when he vacations here. With New Year's Day approaching, though, he told the troops he's inspired "to work harder so I can keep up with you next year."

Perhaps using the "workout stuff" his wife, Michelle, had said she was getting him for Christmas.

The president is known to enjoy a daily workout whether he's at home, traveling, or on vacation. He plays golf practically every weekend in Washington when the weather cooperates, and he swings his clubs just about daily during extended vacations on Martha's Vineyard, Mass., or here in his native Hawaii.

The Christmas visit with the troops has become a tradition for the Obamas during their annual vacation on the island of Oahu. Obama said it's one of their favorite things to do because they get a chance to "say thank you on behalf of the American people."

This year's visit came days after six U.S. service members were killed in a suicide attack in Afghanistan. In his first public comments on the attack, Obama praised the six.

"As we know, when you're deployed overseas, it's tough," Obama said Friday. He said that although his administration has brought home troops from Iraq and Afghanistan, "there are still folks over there every single day, and it's still dangerous, as we saw this past week, where we had some outstanding, brave men and women who were killed."

"So we never take for granted what all of you do for the American people," he said. "You help keep us free. You help keep us strong."

The six service members, including Staff Sgt. Peter W. Taub, 30, a Wyncote native, were killed Monday at Bagram Airfield in Afghanistan, the largest U.S. facility in the country, when a suicide attacker rammed an explosives-laden motorcycle into a joint NATO-Afghan patrol. The Taliban claimed responsibility.