Well, Mario must be in his late 30s (at least).
We have no proof of him doing any excercise between ages 3 and 30.
He probably gained tons of weight back then, and has not lost all of it yet.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS