Shawne Merriman -- I Lost 'Ninja Warrior' ... 'Cause I'm Too Damn Heavy

There's a GOOD REASON you've never seen a 260-pound "Ninja Warrior," 'cause they suck on trampolines ... so says ex-NFL star Shawne Merriman.

Shawne knows from experience -- the former San Diego Chargers linebacker recently appeared on the show ... and started off strong ... only to crash hard when it came time to negotiate the trampoline obstacle.

"260 don't work on trampolines," Merriman says ... "My weight made the trampoline touch the ground."

Gotta give the guy credit -- he actually put together a pretty solid run before bailing on the jump. But SM says if he's invited back next year, he GUARANTEES he'll finish the course.