First, the weaponry. Well, I vaguely recall weaponry. The IG has powerful weapons. I could understand that they may have been for defense only, but since he's a machine and would he had been a research robot they wouldn't have put any weapons on him at all.
Secondly, the dent in the IG head. Amnesia right?!?
I'm just saying that it seems that the Iron Giant is a story about how a war machine landed on earth and would have destroyed the planet had it not been for his amnesia.