"They keep making my costumes smaller!" Dancing With the Stars standout Ricki Lake tells Us of her impressive slimdown on the ABC series. "I've lost at least 15 inches and I feel great!"

As well she should: Thanks to five weeks of marathon dance rehearsals with pro Derek Hough -- and Freshology diet delivery meals -- the 5-foot-4 mom of two has burned off more than 20 pounds.

"I'm working so hard," Lake, 43, tells the new issue of Us Weekly (out Friday). "It's a challenge to really push my body, but it's so fun!"

Having gone down three sizes since September, Lake -- who's slated to host a syndicated talk show in 2012 -- knows the value of her time in the rehearsal studio with Hough.

"I'm using muscles I wouldn't use in the gym or hiking, which is what I love to do," she explains. The best part? "Dancing doesn't feel like working out…even though I'm covered in sweat!"

For more on how Lake scored her sleek physique -- including details on her workouts with fiance Christian Evans -- check out her exclusive video chat with Us Weekly above!