Whitefield preached to tens of thousands of people—sometimes at the one occasion—during the 18th century revival; solid Calvinist theology.
Whether you're a fan of small churches or not, if God decides that he's going to convert a few thousand of your hearers in a village on the strength of one sermon, you're going to end up with a big church.
The wikipedia article on Whitefield is pretty good.
Now imagine how many more he could've reached if he'd had a well-rehearsed music team! ;-)