Its politics and war, if you cant win the war thru game mechanics, then use politics to win the war. As a huge ass nation myself, I personally am pretty tired of people coming on here complaining that large nations have an advantage. Guess what? we should have an advantage, we put a ton of work and effort to become large ass nations, who honestly are already at a disadvantage because we can be hit by a huge number of nations that we cannot touch.
Now if you don't think its fair that people dump military and then are able to double buy and crush you, then join an alliance that has coverage to protect you against that, or make friends with an alliance that can back you up. Once they double buy their NS shoots up and they become vulnerable to people who they cant double buy against. If you beat them down enough times, and not let them steal from you, they will stop hitting you because its not profitable for them to do so.
Stop trying to change the game to make your life easier.