Rob Kardashian Quits Drinking, Looks Thinner at Kim's Bday

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.

We're told Rob's in the best place he's been mentally in a long time too ... fully devoted to raising his daughter, Dream, and making sure she's growing up happily.

Along with this positive development, Rob's also managed to avoid any dust-ups with his baby mama, Blac Chyna, for the past several months ... and that's good for everyone's health.

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.