Jim Keeney, of the National Weather Service, gives us a hailstone scale that's meant to represent hail of increasing size:
pea, marble, dime, penny, nickel, quarter, half dollar, walnut, golf ball, hen egg, tennis ball, baseball, tea cup, grapefruit, softballHm. I suppose it depends upon the size of your marbles and of your grapefruits, but my experience has it that marbles are larger than dimes, and I wouldn't put grapefruit in there as clearly smaller than softballs. And tea cup? Um....
That aside, though, we have this statement from Mr Keeney, commenting on the Iowa "grapefruit-sized stones" not quite making it to the top of the scale:
Right, just a liiiiitle bit bigger, but when we're talking half an inch when it's that big, that's not that different.
Let's over-analyze the physics for a moment. A softball has a diameter of about 4.7 inches, or a radius of about 2.35 inches. If a grapefruit is a half inch smaller, 4.2 inches, that makes its
diameterradius 2.1 inches. What difference does that make for hail? Well, it's true that you'd likely be hard pressed to see the difference in size. But....
The wallop (force) that a hailstone packs is proportional to its mass, which is, in turn, proportional to its volume. Its volume is proportional to the cube of its radius. So the ratio of the volume of the "softball" to that of the "grapefruit" is (2.35/2.1)3, which is about 1.4. In other words, while the "softball" has only 12% more diameter, it has 40% more volume than the "grapefruit". That means it has 40% more mass, and hits your car (or your head) with 40% more force.
That's a lot more than a mere "half an inch when it's that big" makes you think! It's very different.
 Reference formulas:
force = mass * accelerationWe assume that the acceleration is purely due to gravity, which is a constant 32 ft/sec/sec.
mass = density * volumeWe assume the density of the matter composing the hail is constant, regardless of the size of the hailstone.
volume = 4/3 * π * radius3We assume a hailstone is a sphere. It's close enough to that for the purposes of this on-a-napkin calculation.