Ashton Kutcher may have been one of the heartthrobs of the early 2000s ... but he shockingly says he was once fired from a Gucci campaign for being "too fat."

The "That '70s Show" alum shared his story while chatting with Entertainment Tonight recently ... revealing he booked an exclusive campaign for Gucci in his early modeling days and flew all the way to Italy for the shoot. But, when he slipped into his pink Speedo, he says Gucci Creative Director Tom Ford said he was "too fat" and fired him.

Ashton said he was 178 pounds at the time, but didn't take it too personally and understood Ford "had a specific thing he wanted to see." He was about 19 at the time, which would have placed this situation in 1997 or 1998.

He noted he and Ford -- who served as Gucci's creative director from 1994 to 2004 -- later laughed about the situation.

Ashton likely got the last laugh anyway, because he booked "That '70s Show" around the same time and became a household name after the show premiered in 1998.

You win some, you lose some!