I guess they aren't strict about the recruiting of ball-boys. I remember when I applied to be a ball boy at Roland Garros there were physical condition to be met.
They wanted teenagers who weren't taller than 1.75 m (which I find to be disgrace as I was automatically disqualified before entering the physical tests. In my opinion there is nothing wrong with having a ball boy who is taller than most of the players) and there were going to be fitness tests like 40 minutes of aerobic activities and they would also test your ball throwing/receiving skills. Anyways point being, during these physical tests, all the fatties will easily eliminated and you avoid ending up with chubby and clumsy looking ball boys/girls for your tournament. They should do the same in America.