Best of 2011: Nicole Richie's Sexy Boob Job!

Credit: Splash News Online (2)

The B-cup is the new C-cup. 

On the heels of Kate Hudson's 2010 subtle breast enhancement, once flat-chested Nicole Richie, 30, opted for small implants.

The mom of Sparrow, 2, and Harlow, 3, -- with husband Joel Madden, 32, -- unveiled her new cleavage during a bikini-filled vacation with pals in Mexico in September.

"Breast-feeding killed what boobs she did have," a source told Us Weekly of the House of Harlow 1960 designer. "And she wanted to stay sexy for Joel."

One thing the reality star hasn't altered since becoming a mom and a wife? Her enviable style.

"I'm still wearing 5-inch heels," the Fashion Star mentor recently told WWD. "My fashion really hasn't changed at all."