Good point, Cool and Crazy#EN ! Having that in mind I will actually respond instead of ignoring Unknown and challenging his sanity 
I failed to fully understand how the "population destroyed" statistic would work.
I as well am weary of what it would mean for smaller/newer players if suddenly accepting them to bigger kingdoms in order to destroy them would give some type of bonus/nice statistic that some players would try to achieve...
1. We can just as well destroy small players without inviting them to big kingdoms to give us "some type of nice statistic" whatever the f that means.
2. Having in mind what you said, we should actually remove "Top Attacker" statistics, because some bigger kingdoms might kills smaller player troops for "some type of nice statistic".
3. We should also remove "Robbers of the week" statistic, because some big players might attack smaller players for loot to get "Some type of nice statistic".
Did I understand your concerns correctly Unknown ? 
I failed to fully understand how the "population destroyed" statistic would work.
Also, to add to the original topic, I could help you understand that there are 2 ways how to make this calculation:
1. (The hard way): Player X attacks with his army targeting buildings A (level 15) and B (Level 20). If he has enough catapults + troops to destroy them IF THERE WAS NO DEFENCE, but defenders sent some troops and reduced damage to make buildings A (become level 5) and B (Become level 15), the Defenders get: 5 (A levels saved) + 15 (B levels saved) = 20 SAVED BUILDINGS points.
Attacker gets: 10 (A levels destroyed) + 5 (B levels destroyed) = 15 DESTROYED BUILDINGS points.
Since I know this would require full day of coding, which is impossible to expect, I will give you the easy way:
2. (The easy way): Just add 2 sections in statistics, called "Siege units lost as attacker" and "Siege units defended from" where catapult = 2 points, ram = 1 point. I hope I do not need to further explain.
I believe that was the initial proposal. Correct leo#EN(19) ? 