Credit: Casey Rodgers/AP Images

January Jones has cut into her boyfriend’s confidence.

A source tells the new Us Weekly (on newsstands now) that the Mad Men actress’ beau, Saturday Night Live’s Jason Sudeikis, 35, has been doing the BluePrint Cleanse -- a diet consisting of six bottles of juice per day -- because he thinks he looks too puffy when posing for photos next to the slim starlet, 32.

"He put on a few pounds this summer," says the insider. "January is so gorgeous and thin, it’s hard to look good next to her."

And he seems determined to stick with the 1,100-calorie-a-day plan: At an NYC Westville Sept. 29, he met a pal for lunch and drank two bottles of BluePrint Cleanse juices — while the friend ate regular food!