Simon Cowell -- Will Smith's MASSIVE Trailer Is Too Small for Me!

The trailer Will Smith used for "Men In Black III" was too big for New York City ... but it's become too SMALL for the "X Factor" judge Simon Cowell ... because contestants on the show have taken it over!

Sources close to the show tell TMZ ... Simon is using the EXACT same trailer on the set of "X Factor" that Will used back in May. We're told Simon considers it his home away from home and he spends more time there these days than at his home.

According to our sources, not only does Simon relax in it ... but he takes his meetings there and the "X Factor" crew films in it as well. The trailer is 1,200 square feet, but we're told Simon already has his eyes on getting a BIGGER one because so many of the all the crew and passersby who've taken to hanging out there.