So how looked down upon is it to attack and take provinces from a nation that is currently at war because you know it will be easy? Because I can see this being a constant occurrence in multiplayer games that will erase skill. 2 people go to war. They lose their armies. People see their armies are gone and attack in turn for easy provinces. Now you have a few big nations that will eventually eat the other smaller ones that were not able to eat the weakened nations. Seems like it would get boring fast.
To be honest, the first game I started on here ended up with me going toe to toe with the biggest nation out there while I was one of the smallest around, and it led me to pulling in second place while two other big damn nations were duking it out. There's also the inherent tension that comes along with taking apart a small nation between a few bigger ones, and generally, all sides prepare armies for the whole pie and they realize they can't have it.
The other thing is how game theory is a bit more complicated than that. It's good for all big nations to work together for all involved participants, but it's also best for one nation to pull ahead even if it means pulling everyone down with them. It's why I value placing trust in dealing with other nations as well, it's great to get as much as you can to pull forward as fast as you can.
There's also the fact that bigness is not that great. Sorta. Pulling in a huge income and recruiting a lot of troops is all nice, but as the game starts pulling to later stages, nations with higher gem incomes pull in faster. Mother Oak kinda horror marked your nation as a result for example, and I didn't notice you doing anything with that gem income in the meantime.
One last thing, it used to be that games were won by elimination, but in dom 4 they added the thrones of ascension. If you can be particularly sneaky, you can swipe enough thrones to win it doesn't matter if you're losing the war, you can just win while avoiding confrontations that look like trouble.