Based on my reading of Leader Rotation | Solana Docs, it seems that the leader is chosen directly based on the proportion of staked SOL. This might cause clustering of delegated stakes towards larger validators because of the economical benefits for the delegator.
For example, a validator with .000001 of all staked Solana might never be chosen in many epochs, so delegating my stake to such a small validator would be a bad idea. By way of the Law of Large Numbers, I’ll eventually get a proportional return, but in many epochs, I’ll see no returns. Waiting to get some return from a small validator would be like winning the lottery.
However having many smaller validators is better for the Solana system as a whole, since large validators can halt the system. Perhaps instead of choosing the leader based on direct proportion, extra weight should be given to smaller validators. This would incentivize all delegators to spread their stake more evenly among the validators.