As I've said, there is no agreed definition. In any calculation, however, I would expect injury and loss of life to factor. An attack that kills 3,000 is more significant than one that kills 60, surely? Injury to other persons and detrimental effect on the lives of thousands of others, likewise.
You've got the economic cost and damage to property; the NY Times estimated the combined physical damage and economic impact to be around $180bn. An attack that sees entire skyscrapers destroyed is surely more significant than one that leaves a single window smashed?
Diplomatic and political implications would also factor. 9/11 arguably set the tone for the past decade of US diplomacy, led in one way or another to two large armed conflicts, inspired a new generation of radical muslims, etc.
Then there is the wider human impact. The fear, the anger, the hope. Everyone all over the world seems to remember what they were doing the day it happened. People now refer to 'before' and 'after' 9/11 like they are two different eras.
You get the picture. No matter what definition you are working with, the 9/11 attacks were more significant than the 7/7 attacks in every single way.
