COLLEEN COSSITT, a blonde singing-dancing waitress at the Bryson Casino on the Las Vegas Strip, learns best friend FAYE SWITLIK, a dancer at the nearby Purple Pavilion gentlemen's club, is the first to join a showgirl troupe at a rebranded Vegas casino run by club owner VITO CORTEZ. It will be run by wealthy widowed ex-chorus girl ALEXANDRA MAITLAND. Colleen is interested, but Alexandra dismisses her as a candidate since she's only 5-foot-4 3/8 in her stocking feet, well shy of the required minimum height.
Struggling older sister MAUREEN COSSITT says she just met Colleen's high school flame "Big" BILL MESSMER, a 6-foot-7 basketball star turned engineering executive. (Colleen dotes on 10-year-old nephew IVAN BLACK because physical abuse by ex-husband TIM WHITMAN, whose gambling drained her bank account, has left her unable to have children.) At the Bryson, she vents about being short to scientist KESWICK FLETCHER, a longtime customer and close friend who secretly owes Vito $154,000 for an underground gambling debt and is wooing Alexandra for funds. He tells Colleen he’s created a device, the Resizer, that can enlarge bodies proportionally. She visits his lab, a converted trucking warehouse, to see the machine. She asks to have herself enlarged to 6-foot-2 (15 percent growth) so she can become a showgirl. Keswick gives her a night to think it over; each dreams what a taller Colleen would be like (she dreams of wooing Bill, he dreams of getting a kiss from Colleen, whom he secretly adores). The next morning, she gives him the go-ahead for the procedure that afternoon.
As it begins, an enlarged cat escapes her cage, which causes Keswick to accidentally slide the growth rate to its maximum triple scale. Colleen’s body and clothes are consequently made three times larger, and she not only soars to 16-foot-1 1/8, but gets a run in her favorite pair of pantyhose. As the Resizer can only enlarge objects, not reduce them, she’s stuck at that stature, and Keswick fears her condition could be fatal. Faye, Maureen and Ivan come to the lab and stick by Colleen (Ivan loves that his aunt is now "fee-fi-fo-fum" size), unlike Bill, who wants nothing to do with her. She's devastated, but Keswick and aide BELINDA AUSTIN say they'll shelter her in the lab and monitor her health.
Keswick and Belinda unsuccessfully try to restore her over the next few weeks; as time goes by, it becomes apparent her huge size won't kill her, although she discovers maneuvering around the house and lab is difficult. When he and Colleen are alone one evening, they discover they’ve fallen in love. She affectionately calls him “my little Keswick,” and each promises to protect the other. Meanwhile, Alexandra frequents the Purple Pavilion, becoming closer to Tim and Vito; the latter sees a smartphone photo of a giant Colleen from his girlfriend Faye, who tells him it's just a special effect.
The casino’s eccentric billionaire owner, ERNEST SANDERSON (who Vito is blackmailing on behalf of the Boston mob), wants to fire Colleen for a prolonged leave of absence. He finally finds her in the lab, but she gently coerces him into keeping her on the payroll. He signs her to a $1 million deal to headline at his showroom, with Keswick hired as her manager to assure she isn’t exploited. Ernest is also instantly attracted to Maureen. Alexandra visits the lab, discovers Keswick is now in love with Colleen, then calls off her affair with him. She tells Tim and Vito about seeing a giant.
Unveiled to the public with a clumsy, aborted debut for which she publicly apologizes after Ernest threatens to fire her, Colleen soon becomes a hit attraction, a happy, gentle giant beloved by young and old in Sin City. She even performs family matinees on weekends. She repels another romantic challenge from Alexandra and dismisses Tim's pleas for money, saying she won’t lose her “giant virginity” to him. Tim moves in with Alexandra, both vowing revenge, and they align with Vito.
Colleen ends her romance with Keswick and fires him when she learns he owes a six-figure gambling sum, comparing him to her wastrel ex. After he refuses Vito’s offer to use her show as collateral, not letting on he’s no longer her manager, Keswick is kidnapped by Vito’s henchmen. He is absent for several days, and Maureen reminds her sister he truly loves her. A now-lonely Colleen regrets letting him go and cries in her dressing room. Vito -- whose showgirl revue is struggling -- offers to free Keswick if he'll build his syndicate a Resizer, which Alexandra says she'll use to make herself and her showgirls giants.
Faye finds Keswick at Vito’s hideaway, learns of her boss's mob ties and tells Colleen where he’s held. She vows to rescue the scientist, but Faye says Colleen would be spotted as a giant and potentially endanger him. She adds Belinda has altered the Resizer to reduce as well as enlarge, but the scientist's aide warns it hasn't been tested on humans and could jeopardize Colleen's showbiz career. Determined to save Keswick, she goes through with it anyway and is restored to her 5-foot-4 3/8 height.
Colleen, Belinda, Maureen and Faye enter the hideaway and save Keswick. Vito’s gang, including Tim and Alexandra, pursue them, but when they reach the lab, Colleen (who learns to her dismay the new version of the Resizer can’t re-enlarge her) entangles them in her giant lingerie, which she drops from a second-story window.
Vito, Tim and Alexandra escape, entering the lab with Vito holding a gun, which Alexandra retrieves after a fight. She aims it at Keswick and orders him to make herself, Tim and Vito giants, not knowing the machine now only reduces rather than enlarges. Keswick complies, pretending to do it under coercion, and the three are shrunk to one-third scale. A frightened, puny Vito agrees to waive Keswick’s debts after he and the others are hidden from the police. Colleen orders "tiny Tim" not to physically abuse Alexandra, as he had once abused her.
Colleen decides to retire, accepting Keswick’s marriage proposal the night of her farewell show at the Bryson. She closes with a love song to him, just as the shrinking effect wears off and she bursts out of her gown back to triple scale; she and Keswick erroneously think she's willed herself back to giant size. (Vito, Tim and Alexandra return to normal, trapped inside a crib and playpen at the lab.) She tells Keswick if he doesn’t want to marry her now, she’ll understand, but he tells her he’ll keep his promise. “There’s no other man I’d rather lose my giant virginity to,” she privately says to him, then proudly announces her return to performing and their engagement to a delighted, sold-out crowd. Ernest proposes to Maureen for a Cossitt sisters' double wedding, and Keswick gets a movie offer for his re-enlarged fiancee.