Rob stepped outside half naked ... giving us a first glimpse at his bod since he started his diet and fitness regimen. Chyna shot video of him Wednesday afternoon as he peered down at her grilling a ton of grub.
As we first reported ... Rob's already down at least 40 pounds, but it's been hard to notice, since he's still sporting a lot of baggy clothing.
At this rate ... muscle tanks gotta be right around the corner. Spring is coming, Rob!