We could weight the capitulation % to the size of the empires, that might encourage people to war against empires of similar size.
This is a good idea.
Larger empire, higher capitulation %. Creates an inherent check on big empires, limits the breakneck colonization process. No need even to have a comparison for this to work out well. It protects newbies and small empires of all kinds, provides some check to super-massive empires, and gives smaller empires a basic advantage over bigger ones. I see this as kind of how, in BM, smaller realms can run higher tax rates.
This would actually incentivize wars against BIGGER nations... though almost certainly not enough to make it worthwhile.
Also, the more planets you lost, the stronger your defenses would get.
I would envision the capitulation differences as fairly large. A player like uranus, or hierulf, or zootcat might have a planet that, based on its level/population/fleet attacking it/whatever, has a capitulation chance of 30%. An identical planet attacked by an identical fleet, but in an empire with, say, 3 planets, would have a capitulation chance of maybe 5%.