But Messi alone isn't the only one responsible to earn penalties in his team and neither is Ronaldo. Football is a team game. I think instead of focusing on the individual differences between Messi and CR playing style (which we're all aware of) we should look at the way their teams are playing. Barca is often just passing the ball around the box while RM (when CR was there and even now) are crossing into the box, taking long distance shots etc which all improves their odds to get a random push inside the box or a defender playing with his hands,...
But then again, don't think there was that big difference between number of penalties Barca and RM have earned over the years despite different playing style.
Here is a quick comparision from Ronaldo's last three years at Madrid (don't think it makes much sense comparing his stats for Juve playing in a different league)
Penalties in La Liga from 2015-16 to 2017-18
Ronaldo: 15/21
Messi: 11/18
Messi has also given some pens away to Neymar and Suarez so we probably even had more total penalties in that time span.
But this is not really that relevant to your point. It would be better if we'd have stats about how many penalties both Messi and Cristiano have earned by themselves. Don't really know where to find those stats though. I know some sites are counting earned penalties as assists (which doesn't make much sense if a player who earns a pen then also takes it) and I'm too lazy to get into a more detailed analysis in this heat but if someone can provide a good source then I'd be interested in that.