What are you talking about? I agree that he's unlikely to beat Usman, size being one of the reasons why. But he's also shown that he's likely the guy who has the best chance of winning against Usman.
But... what so you mean he got out paced and out cardiod? That literally didn't happen. Colby threw more strikes than Usman and didn't show any signs of being tired, at least not anymore than Usman did. Colby was pushing the pace more than Usman was, and my takeaway from the fight was that Colby has better cardio (albeit probably not that much better) whereas Usman has much more power behind his shots. I think in the future, it's that power that will be the difference, just like in the first fight, but Colby didn't get beat in regards to pace or cardio.
I'm also not sure what's going on with Usman, but he's looked to be getting tired faster in his last couple of fights. He didn't look as fresh as he often does by the end of the Burns fight or the second fight with Jorge.