02/12/2009 at 07:00 PM EST
says she can thank her first child for helping her get back in shape after giving birth to her second.
"You drop some weight just by running after them," she tells PEOPLE.
Watts, 40, most recently gave birth
to son Samuel Kai with boyfriend Liev Schreiber. Their first child, Alexander Pete, was born in July 2007.
The Aussie, who can be seen co-starring with Clive Owen in The International
, exercised and walked a lot during her pregnancy. She adds that nursing also helped her drop the weight.
"I was hungry the whole time and I'm still hungry all the time!"
Reporting by JENNIFER GARCIA