When wrestling, you're grounded, ready for an attack. Sure, a head-butt is unexpected, but you're still fairly prepared for it. The Italian wasn't grounded or expecting it at all. In soccer you don't very often get headbutted when just standing there, 50 feet from the ball. There would be no reason why he would be in a wrestling or fighting stance.
I'm small. But I could lay out a NBA player if he was just standing there, unsuspecting and off-balance. If he was grounded, ready for an attack, I'd hardly scratch him.
BTW-I still think the Italians are stupid divers and that Zidane is cool. He just was stupid and let himself get provoked.
You see crap like that "fall" in pro-fake-wrestling.
His weight was on the left foot and all of a sudden that foot is flying through the air.
The fall was acting.
But it was still a red card play though. There's no debate about that.