So, now that we've pointed out that DPS has diminishing returns and that Armor and HP does not, does that not beg the question: Do defensive stats scale exponentially?
We have a champ D and O with 1000hp and 0 armor, 1 APS and 100 AD. O is attacking D. It takes O 10 seconds to kill D.
If we give D a Warmogs and O a Bloodthirster, D doubles his EHP and O doubles his DPS. O attacks D. It still takes 10 seconds.
If we only give D a Warmogs, it takes O 20 seconds to kill D. A 10 seconds increase.
If we only give O a BT, it takes O 5 seconds to kill D. A 5 second decrease.
O only wins half as much time as D by having his item against someone without items.
So when the defensive champ has stat superiority, he gains more over those he has superiority over than the offensive champ does.
However, this is an extremely gross oversimplification, as there are obviously eight other champions and countless more factors. As you have probably noticed we're a bit past the 5 bruiser meta. I just thought it was an odd effect of the system.