I used to think Chad was smaller than Conor too, but he weighed over 170 for his SUBG match...He's bigger than I expected. Still not quite Conor's size, but not quite the size differential most people think it was.
The biggest problem for Chad in this fight was he just wasn't good enough. He got hit a ton, head, body..when that happens it drains you. It's a big reason Conor loses energy so fast, is because he's constantly will to trade hits because of his chin. He also shot A LOT. Around 10 times in a round and a half, and that's not normal for Chad in an MMA fight. Conor blocked most of them, and all but one Conor eventually got up from. That'll wear any wrestler out eventually.
Still Conor was also visibly tired too from the take downs and eating some big shots himself. Just not as much as Chad, because he was winning the exchanges and exerting himself less as a result. I think people do underrate his cardio though. You have to remember MMA fighters in general don't set a high pace. Nate is obviously the exception, and why it stood out more and made Conor look like he was dying in there for oxygen.