Messi won't be the posterboy for athleticism just because he doesn't look like a one. As being said, any individual sport and he wouldn't succeed so he's hanging there with his team m8s doing all the dirty work for him, so he could have his couple seconds of flair with the ball by being fully recovered compared to the opponent, who's running his lungs out. Ronaldo nowdays ain't any different, but the difference is that with the physique he has, he could easily be playing individual sports with zero support required by anyone - where Messi couldn't.
But we are not talking about any other sport. We are talking about football, and within the context of football, Messi's body type is more ideal than Ronaldo.
Acceleration - Messi's first 5 steps have always been significantly faster than Ronaldo. This allows him to get away from his marker more effectively.
Top speed - very close, both were absolutely rapid in their prime.
Balance - Messi's better balance allows him to ride challenges more easily than Ronaldo without falling down.
Ball shielding - Messi is harder to knock off the ball thanks to his lower center of gravity
Agility - Messi's shorter legs allow him to change direction rapidly. Ronaldo is very fast at running in a straight line but cannot change direction like Messi.
Do you really think that it is merely a coincidence that Pele, Maradona and Messi were all midgets? Or maybe being a midget actually brings some very tangible advantages when playing football?
The biggest advantage Ronaldo has over Messi is his height, leaping ability, and weak foot shots. This makes him a more unpredictable and varied goalscorer than Messi, but not a better player overall.
And I'm sure that Ronaldo can bench more than Messi, but Messi is more effective at using his strength on a football pitch:
Look at the challenges Messi can shrug off while keeping his balance. How many of those challenges would Ronaldo be able to take before he hit the deck? Again, I do not blame Ronaldo for that. His body type (6 foot 2 with heavy upper body muscles) just isn't as suited to football.