Are you aware of the Kingdom Relative Networth modifier that already exists in the gains formula?
There are already significant disadvantages for Kingdoms attacking Kingdoms of lower strength while at the same time the smaller Kingdoms have no penalties for retaliating (i.e. no negative modifier on gains).
I don't want you to think I'm just blowing off your statement, but what would your suggestion be to resolve the issue you're stating? Keeping in mind, large kingdoms hitting a small kingdom can receive as much as a -34% penalty in gains just from Kingdom nw difference.