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They are two completely different things.
The Hurling power allows you to lift and throw an item of a specific size (or smaller). The damage is based on what item you throw. Even if you have Greater Hurling a fist-sized stone still does the damage of a fist-sized stone. There is nothing virtual in it.
The Shikigami style instead gives a virtual increase in size that increase the damage. That doesn't stack with Hurling because there is nothing to stack.
The Barbarian Hurler with Greater Hurling and Shikigami style can pick up an item that is a size larger than himself and hurl it. The base damage is determined by the size of the object used. Then the Shikigami style increases the damage by one or more categories, based on the number of feats you have.