Imagine one of my players casts the growth spell on himself. Ok, he becomes twice as big, and so what?
Does that make him stronger? How much? (if you follow the laws of physics, he would need to be much stronger, or just collapse on himself, and since his weight has been multiplied by 8, but his muscle section only by 4, he would have a difficult time). Are there any other stat changes?
His he considered a large creature for receiving critical? Is the size of its attacks increased?