| mplindustries |
"A hit deals direct hit damage to the target, and splash damage to all creatures within 5 feet of the target. If the target is Large or larger, you choose one of its squares and the splash damage affects creatures within 5 feet of that square."
So, you hit the Large (or larger) creature with the direct hit damage. You pick one square the creature occupies and anyone within 5' of that square takes the splash damage. The creature does not take direct hit damage AND splash damage, nor does the splash area get larger because the creature is larger.