David Spade -- Hey Reese Witherspoon ... You're NOT THAT FAMOUS!

Reese Witherspoon learned the hard way -- if you have to tell someone, "Do you know who I am?" ... you're NOT as famous as you think you are, this according to David Spade.

Spade was leaving Boa Steakhouse in Hollywood last night when we asked if there's ever a good time for a star to drop the "I'm a really big deal" schtick ( a la Reese) ... and the "Rules of Engagement" star gave it to us straight.

"If you have to explain to someone you're famous, then you're technically not that famous."