I cannot emphasize what a terrible idea it is to have two inputs to the gains formula. It is important that provinces actually be able to attack each other in this game, and reducing the chance that similarly-sized ones can do so is a terrible idea. But beyond that, it's important that players be able to understand what is going to happen when they make an attack. This game is math-centric enough already.

To prevent bottomfeeding on provinces much smaller than you, it would be sufficient to simply raise the networth given by land. Basically every age for years now the % of nw coming from land/wizards/thieves has dropped, as troop networths keep getting inflated more and more. This is the reason why you see provinces hitting others 1/2 their size now when it never happened before.