Rob Kardashian's been using his time out of the public eye to focus on his health and drop some pounds ... and a lot of it's thanks to saying no to alcohol.
The Kardashian bro was spotted in Kim's Instagram Story from her birthday celebration Monday night, clearly looking a lot leaner than he used to.
Sources close to Rob tell TMZ ... he quit drinking recently and has been focusing on making better diet choices -- including cutting down on fast food. We're told he's also exercising on the reg and playing a lot of basketball to get in shape.
All of this combined has led to him shedding a lot of weight -- our sources say in the ballpark of 20 lbs. ... maybe more compared to a couple years ago.
As for Rob's sister's 39th birthday party ... it was pretty sweet. The fam got a "home cooked meal" from Carousel Restaurant -- which cooks up Middle Eastern cuisine -- and Kim was showered with gifts.
The best one, though, came in the form of a million-dollar donation from Kanye in her honor ... to support some of her favorite charities.