Okay I see what you mean now.
That's easy enough to alter - make the size of your country multiple the cost of any relationship, in addition to the previous larger-nations-cost-more as a target change.. So if you're much larger, any given relationship will cost you a lot more than it would for a smaller nation.
So say you have A, size 10 base tax, and B, size 10 base tax, and C, size 50 base tax, and D, size 50 base tax.
A - B would cost (made up numbers) 25% relations for A
A - C would cost 50% relations (higher because C is large and valuable) for A
C - D would cost 100% relations (higher because D is large and valuable and because C itself is large) for C
Of course, then the whole idea set starts to get relatively complex and potentially inelegant. But at least it would be something.
Yeah that sounds perfect.
I agree it gets a bit more complex. Then again I look at some mechanics in this game, like production goods creating trade value which leads into production income (except when it's tariffs), with multiple overlapping building types that manipulate this income by modifying different points in one of a couple of different process flows, using varying modifiers which might be additive or multiplicative, and resulting in a simple question like "how much will this building increase my income by?" requiring an Excel spreadsheet.. and I think complex is probably not a problem