I understood the OP to have said something along the lines of if we have a fixed cost per object then we should bias towards smaller objects if we want to minimize that cost.
And totally legibility and/or simplicity. I'll take something I can reason about and maintain over something more complicated just to eek out a tiny better hit ratio. That said, if you're caching at scale your 0.05% hit ratio can be a big deal.
As a matter of personal taste/opinion I also shy away from close loop systems. Feedback makes things complicated in non-intuitive ways. Caffeine seems neat in terms of using feedback to adjust the cache to the workload - as always test with your workloads and pick what is best for your situation.
