Well, I agree that the scale is accurate in regards to orcs and other infantry, but the fact that Grond is now twice as tall as the gates of Minas Tirith seems to stick out to me.
Also, I just played a skirmish in Minas Tirish and Grond wasn't able to attack anything. I could only move it around, there was no red cross to indicate attacking. Would there be a possible explanation for this?