Anna Faris explains how her son helped her shape up

Anna Faris credits her son Jack for her newly toned arms.

“My arms have never been toned before and now they are because he’s huge,” the 36-year-old actress told People magazine.

“He’s got a huge head, a huge body. He’s like 60 pounds,” she jokes.

Faris prematurely gave birth to Jack 13 months ago. Fortunately, he has grown into a healthy sized toddler and is "so active."

Speaking of weight, the "Mom" star said she wished her husband Chris Pratt -- who recently slimmed down for his role in Marvel's "Guardians of the Galaxy," would gain some of it back.

“It’s so annoying,” she said. “I can’t wait for him to come back from shooting. I do [miss his old size] a little bit.”

In July, Pratt shared a picture of his new six pack, but Faris is confident that he'll come back to his former size sooner than later.

“We both love to eat and drink, so I’m sure [the weight won't stay off]. You have to live life sometimes, you know?”