Again, there's skill involved yes. No question.
But if he kills himself to make weight (or at least tries and gets close) that fight very likely plays out differently. Brooks had to make weight, he dehydrates himself. Wins the first 2 rounds (most likely), then gets tooled in the 3rd. To think Cowboy not having to dehydrate himself and go through all of that didn't potentially affect the outcome...c'mon. And had he lost, we wouldn't be talking at all about how he managed to gain and keep the clinch at times during the fight and how amazing his grappling looked. We'd be maybe saying "Well, as the bigger guy he still did okay in the clinch and the grappling. Not enough, but he didn't get owned at least." Instead, he missed weight by a bunch and then won so the narrative changes into how much skill he showed to pull off the W.
No way was size the only factor. There was skill, yes. His grappling has improved. The big miss on the weight cut and all that goes with it played a big factor too though.