"He has some nice ground skills, too, but I feel like I’m on another level, and (with) size and strength"
"I feel that’s a fight I win nine out of 10 times.”
It's not about weather he can or cannot, he can, anyone in MMA can be beat. It's the way he presents it, arrogance and confidence are similar but not the same. If he believes in himself he should say he can win, but he dumbs up the message up by speaking too much on it.
I expect most fighters to give a reply of confidence when presented similar opportunities to speak on such matter, but what you say dictates how you come off to others. He could say a lot of things like "Pettis is a good tough champ, it would be a great challenge and I think I find a way to win focusing on and utilizing my best attributes." Instead he gives us a silly response suggesting Pettis isn't even on his level. Get real Diego.
I really don't see what's wrong in another fighter claiming that they think they can beat the champion 9/10 times. That's confidence, not arrogance. Stating that he thinks his ground game and strength is on another level also, that's just supreme confidence (whether it's true or not is irrelevant, this is about a fighters mental state of mind in challenging for the title, no shred of doubt can be present in the mind).
It would be arrogant if Diego started tarnishing Pettis' ability, saying he doesn't deserve to be champion or that he sucks and then boasting about his own skills. That would be arrogant. Take a look at Chael Sonnen if you want to see arrogance.
I beat the champ 9/10 is not an arrogant statement, it's a statement I'd expect to hear from ANY fighter with a true belief in themselves.
Now, whether that's actually accurate or not is irrelevant, of course Diego doesn't beat Pettis nine times out of ten, but there's nothing wrong in him believing that he can and stating that he can.