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.
Have an example of this "limited market" and when it impacted the pool??
From what I can tell we still mine lots of Doge coin.