Newton's Second Law. If you apply a force to something, it acclerates. If that something is light, it accelerates fast, and if it is heavy, it accelerates less. A football weighs a LOT more than a baseball (comparitively). Since the average human arm can only deliver x amount of force, the acceleration depends on the ball. Assuming the football is twice as heavy as the baseball, then the acceleration can be found like this:
Acceleration = Force / Mass
so for the baseball of mass m:
and for the football of mass 2m:
So you can see, that since there is less acceleration the ball must therefore end up going at a slower speed.