Round balls are under bore size, wrapped in a cloth patch, and expand when fired to grip the rifling - soft lead is best. Maxi balls have a bore sized base and groove sized nose, so the nose is forced into the rifling when loading, the base expands when fired to grip the rifling - soft is best. Minie bullets are under bore size and the hollow base expands when fired to grip the rifling - soft is best.
However, round balls don't expand that much, and with a tight patch I've used range, wheel weight, and even linotype lead.
The most important reason for soft lead is to get the dang ball screw into the ball you loaded with no powder under it