I think people confuse an excuse with an explanation in these parts. An excuse is "I lost because of [insert atypical situation here]. On any other day, I would have won." An explanation is "I lost because of [insert reason here]".
I'm not a huge McGregor fan, but come on, he didn't make excuses. He said the guy was bigger and he wasn't able to knock him out easily because of that. If anything, he's admitting his power is more limited than he originally thought. And because of that he gassed because he threw hard all the time thinking he'd eventually get a knock out, only to find out a bigger guy can't be so easily knocked out.