When Tara Rachel Benson went out on a recent night to an album release party in Los Angeles, she put on her makeup, a tight-fitting Herve Leger dress, stiletto heels—and a pair of padded panties.

"It's part of the whole outfit," says Ms. Benson, a 25-year-old assistant to a music manager. Wearing the Booty Pop brand of underwear, which contain egg-shaped foam pads to plump up the posterior, "I look better, I feel better, and as a result, I act better," she says.

For...