Pool choice?

Noob question, apologize if answer is obvious or stupid.. Joigning large pool is bad because we need decentralization. But as a miner, what are the pro and con of joigning a small pool?

If I understand well, a small pool as little chance to find blocks but if they do, the reward is split in less people (because it's a small pool). On the other hand large pool with more hashrate may find more blocks but with rewards split into many miners….

How can we estimate the miner reward as a function of pool size.

Why there is no mechanism that increase difficulty for large pool (thereby enforcing decentralization?)