He's 6'7, so even if he looks much skinnier on Instagram, he still weighs about 300 pounds. The real issue is whether or not his belly (or lack thereof) functioned as a sort fleshy counterweight that allowed him to execute his mechanics properly.
At least one "talent evaluator" thinks CC Sabathia developed balance problems after losing weight heading into the 2013 season. Significant weight loss could throw off his delivery. Or it might not matter at all. But since he's under contract for $71 million through 2016, the Yankees might be a lot better off if CC looked like he did when this picture would've been cool.