Thanks. Yes, diminishing returns is a core concept in BattleMaster. Skills are a great example: The better your skills are, the more it takes to grow them. It's actually the reverse of exponential growth, many growth figures in BM are inverse power functions like that.
And that's necessary, because BM is set in a finite world. Most other games of this kind are set in an infinite world - one where you can basically grow forever. In BM, the income of the land is a fixed factor, and you can't grow it (at least not permanently). That one design decision sets BM apart from almost every other strategy, world-building game out there.
And I think it was a good decision to make.
