Dancin’ BRISTOL the Pistol PALIN packs on 15 pounds despite all the exercise as she grows EVEN bigger from binge-ing on reams of caloric crap!

Because the dance that Bristol Palin is practicing most is the Junk Food Hop!

Sarah Palin‘s daughter has packed on an astonishing 15 pounds since she began competing on Dancing with the Stars on Sept. 20, says a close friend.

The single mom, who is 5-foot-5, started the show at about 120 pounds, but is now around 135. In a photo snapped last year, she was still in great shape.

How is it possible to gain so much weight so fast while burning loads of calories on the dance floor?

"Bristol’s been eating tons of fast food," revealed her friend. "She used to eat very healthy, but now she says she just doesn’t have time."

The 20-year-old Alaskan was recently photographed at McDonald’s, and has been hitting other fast-food joints as well, according to her pal. "

Friends back home in Wasilla look at her on TV and can’t believe how big she’s gotten. The weight has really shown up in her face.

"Even though she gets so much exercise on the show, she’s not eating right."

Despite her talentless two-left feet mastery rivaling KATE GOSSELIN’s pitiful display last season, Bristol has been untouchable on DWTS so far.