You mean that the pressure from an explosion follows the inverse cube of the distance, so even increasing the explosive power a lot wouldn't matter that much because the distance to damaging-pressure (for whatever threshold you choose) would only increase with the cube root of the factor it was increased by?
How do you get 10% more? Based on that logic I'd expect the damage radius to be 1.6x the size for a 5x larger explosion, which would mean 2.5x the area inside a damage-threshold pressure contour.
Mostly because ammonia is shitty explosive. So it just wouldn't explode that efficiently if we make the pile bigger. The fact that it is 5 times bigger doesn't mean that it would release 5 times the energy in a detonation.
How do you get 10% more? Based on that logic I'd expect the damage radius to be 1.6x the size for a 5x larger explosion, which would mean 2.5x the area inside a damage-threshold pressure contour.