The 19-year-old Michigan native - who graces the cover of the 2012 Sports Illustrated Swimsuit Issue - told Esquire that she didn't fit the traditional image of a model as a teenager.
"I was very short, actually, so casting said, 'No, you're too short'," Upton recalled to the magazine.
The star continued: "Scouting a model is different - it's not like you're looking for the next sex bomb, the next Victoria's Secret model. They're looking for certain features. But I was too short and I was only 12. And then I grew."
Upton got her break posing for Garage and Dooney & Bourke and has gone on to model in several major campaigns.
She follows in the footsteps of Christie Brinkley, Tyra Banks and Petra Nemcova in appearing on the cover of the 2012 Sports Illustrated Swimsuit Issue.
> Sports Illustrated Kate Upton is glorified Page 3 girl, says stylist
Watch Kate Upton cameo on this past weekend's Saturday Night Live below (US only):