David Beckham -- Ugly Mansion in 'NFL on Fox' Sketch Is NOT My Real House

David Beckham doesn't really live in the monstrosity of a mansion featured on the "NFL on Fox" pregame show this weekend -- Dave's house is WAYYY bigger ... TMZ has learned.

Beckham appeared in a comedy sketch with the new "NFL on Fox" comedy guy Rob Riggle ... and in the sketch, Rob joked that he broke into "some football player's home" to throw a raging party. Enter the punchline when Dave said the house was his. Funny stuff.

The home was gigantic ... and SOOO gaudy ... with a massive pool .. a fancy dining room decked out in gold ... and a tacky master bedroom.

But sources close to Beckham tell us ... the footage was NOT shot in Dave's real-life Beverly Hills mansion ... but rather a rented out Bel Air home made to look like Casa de Becks.

Dave's actual Bev Hills home DWARFS the fake pad ... measuring 13,000 square feet with 6 bedrooms, 9 bathrooms and a HUGE backyard complete with palm trees and views of the ocean.

Not bad for some football player.