Thunderstorms are caused by local columns of rising air called updrafts. Often, these strong updrafts are enough to overcome the force of gravity long enough to send raindrops up into the colder tops of the clouds where they freeze. Tiny hail usually melts on the way down, but small hail (the size of peas or dimes) is a relatively common occurrence during summer. Larger hail is obviously far more damaging but, fortunately, also rarer.
Very large hail is very rare. Nevertheless, almost every summer, we get a report of hailstones the size of baseballs or even softballs falling somewhere in the Dakotas or Minnesota. Hail this big requires an updraft of around 100 mph or higher. Such an updraft is difficult to maintain and so these super large hail stones are rare in any one location. Fortunately, only a very few of us will see one in our lifetimes.