I don't get how some of you are figuring the guy with 600+Mh/s is taking money from the rest of us. This is a proportional pool. He earns his share with his hashrate as do we all. It makes no difference if he is mining or not. You will still get the same amount proportional to how many shares you submit.
if you take in the account the limited market for smaller and more profitable altcoins, then yes, the bigger we are, the lesser the profits will be, since lesser coins have small market depth so we can't mine (a lot) of them, so we must go for the bigger, less profitable ones, hence the bigger we are, the lesser is our profitability.