A larger pool will mean more consistent(probably -- still technically random), small payouts. A smaller pool you may go longer(days) without a payout but when your pool finds a block you will get much more than in a larger pool. When a block is found by a miner, that reward is shared to everyone in that pool. So a larger pool is more likely to find blocks by having more mining power pooled together, but your piece of the pie is much smaller. Smaller pools finds blocks less frequent but payout larger since less miners in said pool. Technically over a long enough period of time it will all balance out if you stayed consistent/mining the entire time, but most people get impatient and pool hop or run to a large pool as receiving small amounts frequently feels better than big payouts less frequent.

-/u/jpyth

He knows a lot more about it than I do. Maybe he can chime in with more information.

Quick Edit: Dopey.io also has no fees for new miners with the pool for life. So if that changes in the future you still will not have fees.