Al Sharpton -- Lose Weight, Tubby! Fat-Shames Kenan Thompson

If Kenan Thompson ever wants to host a late night show, Reverend Al Sharpton says he better drop some major poundage because nobody wants to fall asleep to a puffy porker.

Our photog asked Sharpton which black comedian he'd like to see snag a hosting gig, when out of nowhere he begins blasting Thompson's weight.

It's a strange response from Al -- who was once 305 lbs. himself, but now thinks Kenan is so big ... just seeing him gives people the munchies.

Ooooh-weeee ... what up with that, Al? Check it out.