Alley was set to perform Tuesday night in the costume she wore on the "Dancing With The Stars" premiere back in March. But she said the costume had to be taken in 38 inches for her to wear it again when she reprises her opening-night dance, the Cha-Cha, during the finale.
The actress, who is paired with professional dancer Maksim Chmerkovskiy, has been noticeably shrinking week after week. When asked Monday night how much weight she has lost during her two and a half months on the show, Alley remained coy.
"It was like, keep whittling, whittling, whittling," she said. "And the other thing that's really more significant to me - I mean, I love the whittling, I like being skinny - I'm really strong and really agile and it gave me a new life."
During her freestyle dance on Monday night, Alley showed off her slimmer physique when she ripped off her dress to finish her performance in a skin-tight bodysuit. After the performance, Judge Carrie Ann Inaba told Alley that she has become "the new poster child for life as it should be lived at 60 years old."
Alley's costumes have gotten progressively smaller and skimpier as she has continued on the show.
"We are constantly taking in and altering Kirstie's costumes," said designer Randall Christensen. "Isn't she looking incredible?!"
So who will win the mirror ball trophy on "Dancing With The Stars" season 12? Will it be Alley, actress Chelsea Kane or football star Hines Ward? The new champion will be named on Tuesday night's finale at 9 p.m. on ABC7.
The Associated Press contributed to this report.