I had thought it was a pretty rare (once an age) problem where a top kingdom declared on someone tiny. If it is more common than that though, then a 50% ratio is at least playable. Nobody uses multis to trigger farmwars with kingdoms 60% their size. One other idea I had though was that button hostility level could depend on the nw range; i.e., a kingdom 1/2 your size would take 2x as many hits (15 trads instead of 7.5) to go to hostile.
OOW fights massively favor a significantly smaller kingdom, because they get +50% gains on all their hits. It's really as simple as that. When you talk about kingdoms getting chained down in war, they end up with small provinces that anyone can pick on. Other than razekilling (which is absurdly inefficient, at 5% per hit) there is nothing you can do to stop it. It's impossible to explore up small provinces to get them out of range, since explore costs are x2 for chained provinces versus what they were two ages ago and also x2 in postwar. Kingdoms running cows contributes to the problem but since there is no other way to use pool that's not going to stop under current rules. One other idea I had here was to have KNBG (kingdom networth-based gains) by symmetrical, so hitting a kingdom 1/2 your size is -1/3 gains for both sides. That would then favor a kingdom bottomfeeding on one 80-90% their size more than it does now, but in conjunction with fixing province-nw-based-gains to make them symmetrical that could be balanced. Maybe.
Removing GBP oow would be a really, really bad idea. Removal of gbp protection is about 75% of the war effects, so you'd already basically be in war once that happened. Meanwhile raze is available oow (only 5%, but still deadly without gbp) while it is not in war. And the smaller kingdom can chain while the bigger cannot. The result is an incredibly destructive chain/raze/razekill fight, with none of the benefits you get from actually warring.